diff --git a/.babelrc b/.babelrc
index f3b952604a7d..f9a07bc9ff29 100644
--- a/.babelrc
+++ b/.babelrc
@@ -1,6 +1,5 @@
 {
-  "experimental": true,
-  "playground": true,
-  "loose": true,
+  "stage": 0,
+  "loose": ["all"],
   "blacklist": ["es6.tailCall"]
 }
diff --git a/package.json b/package.json
index 7dc776128946..2eb81dd5532e 100644
--- a/package.json
+++ b/package.json
@@ -59,7 +59,7 @@
     "user-home": "^1.1.1"
   },
   "devDependencies": {
-    "babel": "4.7.13",
+    "babel": "5.1.13",
     "browserify": "^9.0.8",
     "chai": "^2.2.0",
     "eslint": "^0.18.0",
diff --git a/src/acorn/index.js b/src/acorn/index.js
new file mode 100644
index 000000000000..1bae528e73e4
--- /dev/null
+++ b/src/acorn/index.js
@@ -0,0 +1,3 @@
+export * from "./src/index";
+import "./plugins/flow";
+import "./plugins/jsx";
diff --git a/src/acorn/package.json b/src/acorn/package.json
index c2cc60b77c15..4067160963ec 100755
--- a/src/acorn/package.json
+++ b/src/acorn/package.json
@@ -2,7 +2,7 @@
   "name": "acorn",
   "description": "ECMAScript parser",
   "homepage": "https://github.com/marijnh/acorn",
-  "main": "src/index.js",
+  "main": "index.js",
   "version": "1.0.0",
   "engines": {
     "node": ">=0.4.0"
diff --git a/src/acorn/plugins/flow.js b/src/acorn/plugins/flow.js
index 4f57bf078d56..1c4fdcce3874 100644
--- a/src/acorn/plugins/flow.js
+++ b/src/acorn/plugins/flow.js
@@ -1,4 +1,4 @@
-var acorn = require("..")
+var acorn = require("../src/index")
 
 var pp = acorn.Parser.prototype
 var tt = acorn.tokTypes
diff --git a/src/acorn/plugins/jsx.js b/src/acorn/plugins/jsx.js
index 2d249a17a2d8..cf6c327fc052 100644
--- a/src/acorn/plugins/jsx.js
+++ b/src/acorn/plugins/jsx.js
@@ -1,4 +1,4 @@
-var acorn = require("..")
+var acorn = require("../src/index")
 
 var tt = acorn.tokTypes;
 var tc = acorn.tokContexts;
diff --git a/src/acorn/src/index.js b/src/acorn/src/index.js
index f4fabd338c12..4aa982fa19b6 100755
--- a/src/acorn/src/index.js
+++ b/src/acorn/src/index.js
@@ -38,9 +38,6 @@ export {isIdentifierChar, isIdentifierStart} from "./identifier"
 export {Token} from "./tokenize"
 export {isNewLine, lineBreak, lineBreakG} from "./whitespace"
 
-import "../plugins/flow";
-import "../plugins/jsx";
-
 export const version = "1.0.0"
 
 // The main exported interface (under `self.acorn` when in the
diff --git a/src/babel/api/browser.js b/src/babel/api/browser.js
index 1a8ed4630c35..d90ec4ec7142 100644
--- a/src/babel/api/browser.js
+++ b/src/babel/api/browser.js
@@ -11,7 +11,7 @@ transform.run = function (code, opts = {}) {
 };
 
 transform.load = function (url, callback, opts = {}, hold) {
-  opts.filename ||= url;
+  opts.filename = opts.filename || url;
 
   var xhr = global.ActiveXObject ? new global.ActiveXObject("Microsoft.XMLHTTP") : new global.XMLHttpRequest();
   xhr.open("GET", url, true);
diff --git a/src/babel/generation/buffer.js b/src/babel/generation/buffer.js
index 2e909cf5256b..bb47b6a753ea 100644
--- a/src/babel/generation/buffer.js
+++ b/src/babel/generation/buffer.js
@@ -77,7 +77,7 @@ export default class Buffer {
       return;
     }
 
-    removeLast ||= false;
+    removeLast = removeLast || false;
 
     if (isNumber(i)) {
       i = Math.min(2, i);
diff --git a/src/babel/generation/index.js b/src/babel/generation/index.js
index ef4bf56f622b..7e35ed694a83 100644
--- a/src/babel/generation/index.js
+++ b/src/babel/generation/index.js
@@ -12,7 +12,7 @@ import * as t from "../types";
 
 class CodeGenerator {
   constructor(ast, opts, code) {
-    opts ||= {};
+    opts = opts || {};
 
     this.comments = ast.comments || [];
     this.tokens   = ast.tokens || [];
diff --git a/src/babel/generation/node/whitespace.js b/src/babel/generation/node/whitespace.js
index 1db5d42d4295..6befad891c74 100644
--- a/src/babel/generation/node/whitespace.js
+++ b/src/babel/generation/node/whitespace.js
@@ -16,7 +16,7 @@ function crawl(node, state = {}) {
   } else if (t.isFunction(node)) {
     state.hasFunction = true;
   } else if (t.isIdentifier(node)) {
-    state.hasHelper ||= isHelper(node.callee);
+    state.hasHelper = state.hasHelper || isHelper(node.callee);
   }
 
   return state;
diff --git a/src/babel/tools/resolve-rc.js b/src/babel/tools/resolve-rc.js
index 78fe34e14e91..eb3b0334cd1a 100644
--- a/src/babel/tools/resolve-rc.js
+++ b/src/babel/tools/resolve-rc.js
@@ -33,7 +33,7 @@ export default function (loc, opts = {}) {
       var json;
 
       try {
-        json = jsons[content] ||= JSON.parse(stripJsonComments(content));
+        json = jsons[content] = jsons[content] || JSON.parse(stripJsonComments(content));
       } catch (err) {
         err.message = `${file}: ${err.message}`;
         throw err;
@@ -47,7 +47,7 @@ export default function (loc, opts = {}) {
           var c = a.slice(0);
           for (var v of b) {
             if (a.indexOf(v) < 0) {
-              c.push(v); 
+              c.push(v);
             }
           }
           return c;
diff --git a/src/babel/transformation/file/index.js b/src/babel/transformation/file/index.js
index d9df3fdb92cc..159dd396fe5d 100644
--- a/src/babel/transformation/file/index.js
+++ b/src/babel/transformation/file/index.js
@@ -134,7 +134,7 @@ export default class File {
       if (optionParser) val = optionParser(key, val);
 
       if (option.alias) {
-        opts[option.alias] ||= val;
+        opts[option.alias] = opts[option.alias] || val;
       } else {
         opts[key] = val;
       }
@@ -349,7 +349,7 @@ export default class File {
   }
 
   addImport(source: string, name?: string, type?: string): Object {
-    name ||= source;
+    name = name || source;
     var id = this.dynamicImportIds[name];
 
     if (!id) {
@@ -361,7 +361,7 @@ export default class File {
       declar._blockHoist = 3;
 
       if (type) {
-        var modules = this.dynamicImportTypes[type] ||= [];
+        var modules = this.dynamicImportTypes[type] = this.dynamicImportTypes[type] || [];
         modules.push(declar);
       }
 
@@ -379,7 +379,7 @@ export default class File {
   attachAuxiliaryComment(node: Object): Object {
     var comment = this.opts.auxiliaryComment;
     if (comment) {
-      node.leadingComments ||= [];
+      node.leadingComments = node.leadingComments || [];
       node.leadingComments.push({
         type: "Line",
         value: " " + comment
diff --git a/src/babel/transformation/helpers/define-map.js b/src/babel/transformation/helpers/define-map.js
index d6acd8e93eba..6d5efda84948 100644
--- a/src/babel/transformation/helpers/define-map.js
+++ b/src/babel/transformation/helpers/define-map.js
@@ -15,7 +15,7 @@ export function push(mutatorMap, node, kind, file) {
 
   //
 
-  map._inherits ||= [];
+  map._inherits = map._inherits || [];
   map._inherits.push(node);
 
   map._key = node.key;
@@ -25,7 +25,7 @@ export function push(mutatorMap, node, kind, file) {
   }
 
   if (node.decorators) {
-    var decorators = map.decorators ||= t.arrayExpression([]);
+    var decorators = map.decorators = map.decorators || t.arrayExpression([]);
     decorators.elements = decorators.elements.concat(node.decorators.map(dec => dec.expression));
   }
 
diff --git a/src/babel/transformation/helpers/remap-async-to-generator.js b/src/babel/transformation/helpers/remap-async-to-generator.js
index 56faaeb7ab32..93c5d320dc38 100644
--- a/src/babel/transformation/helpers/remap-async-to-generator.js
+++ b/src/babel/transformation/helpers/remap-async-to-generator.js
@@ -20,7 +20,7 @@ var referenceVisitor = {
   enter(node, parent, scope, state) {
     var name = state.id.name;
     if (t.isReferencedIdentifier(node, parent, { name: name }) && scope.bindingIdentifierEquals(name, state.id)) {
-      return state.ref ||= scope.generateUidIdentifier(name);
+      return state.ref = state.ref || scope.generateUidIdentifier(name);
     }
   }
 };
diff --git a/src/babel/transformation/helpers/replace-supers.js b/src/babel/transformation/helpers/replace-supers.js
index a57da1569f48..3e43881227ca 100644
--- a/src/babel/transformation/helpers/replace-supers.js
+++ b/src/babel/transformation/helpers/replace-supers.js
@@ -218,7 +218,7 @@ export default class ReplaceSupers {
       return this.setSuperProperty(node.left.property, node.right, node.left.computed, getThisReference());
     } else {
       // super.age += 2; -> var _ref = super.age; super.age = _ref + 2;
-      ref ||= path.scope.generateUidIdentifier("ref");
+      ref = ref || path.scope.generateUidIdentifier("ref");
       return [
         t.variableDeclaration("var", [
           t.variableDeclarator(ref, node.left)
diff --git a/src/babel/transformation/index.js b/src/babel/transformation/index.js
index 8d244aa51a63..4e6494e8f169 100644
--- a/src/babel/transformation/index.js
+++ b/src/babel/transformation/index.js
@@ -60,7 +60,7 @@ import rawTransformers from "./transformers";
 each(rawTransformers, function (transformer, key) {
   var namespace = key.split(".")[0];
 
-  transform.namespaces[namespace] ||= [];
+  transform.namespaces[namespace] = transform.namespaces[namespace] || [];
   transform.namespaces[namespace].push(key);
   transform.transformerNamespaces[key] = namespace;
 
diff --git a/src/babel/transformation/modules/_default.js b/src/babel/transformation/modules/_default.js
index 6d237a54c9a0..9c511b07ff4a 100644
--- a/src/babel/transformation/modules/_default.js
+++ b/src/babel/transformation/modules/_default.js
@@ -166,7 +166,7 @@ export default class DefaultFormatter {
   }
 
   _addExport(name, exported) {
-    var info = this.localExports[name] ||= {
+    var info = this.localExports[name] = this.localExports[name] || {
       binding: this.scope.getBindingIdentifier(name),
       exported: []
     };
diff --git a/src/babel/transformation/modules/common.js b/src/babel/transformation/modules/common.js
index 0679e2ed76eb..3f43eda36fbd 100644
--- a/src/babel/transformation/modules/common.js
+++ b/src/babel/transformation/modules/common.js
@@ -114,7 +114,7 @@ export default class CommonJSFormatter extends DefaultFormatter {
       uid = this.scope.generateUidBasedOnNode(node, "import");
     }
 
-    uid ||= node.specifiers[0].local;
+    uid = uid || node.specifiers[0].local;
 
     var declar = t.variableDeclaration("var", [
       t.variableDeclarator(uid, call)
diff --git a/src/babel/transformation/modules/system.js b/src/babel/transformation/modules/system.js
index 74e5d022f317..4813084446b2 100644
--- a/src/babel/transformation/modules/system.js
+++ b/src/babel/transformation/modules/system.js
@@ -77,11 +77,11 @@ var runnerSettersVisitor = {
 
 export default class SystemFormatter extends AMDFormatter {
   constructor(file) {
+    super(file);
+
     this.exportIdentifier = file.scope.generateUidIdentifier("export");
     this.noInteropRequireExport = true;
     this.noInteropRequireImport = true;
-
-    DefaultFormatter.apply(this, arguments);
   }
 
   _addImportSource(node, exportNode) {
diff --git a/src/babel/transformation/transformer.js b/src/babel/transformation/transformer.js
index 361a329078a3..f895b285f887 100644
--- a/src/babel/transformation/transformer.js
+++ b/src/babel/transformation/transformer.js
@@ -39,7 +39,7 @@ export default class Transformer {
     //
 
     this.handlers = this.normalize(transformer);
-    this.opts     ||= {};
+    this.opts     = this.opts || {};
     this.key      = transformerKey;
 
     //
diff --git a/src/babel/transformation/transformers/es6/block-scoping.js b/src/babel/transformation/transformers/es6/block-scoping.js
index 4c2bea586a7e..1b81d95232f6 100644
--- a/src/babel/transformation/transformers/es6/block-scoping.js
+++ b/src/babel/transformation/transformers/es6/block-scoping.js
@@ -14,7 +14,7 @@ function isLet(node, parent) {
   if (isLetInitable(node, parent)) {
     for (var i = 0; i < node.declarations.length; i++) {
       var declar = node.declarations[i];
-      declar.init ||= t.identifier("undefined");
+      declar.init = declar.init || t.identifier("undefined");
     }
   }
 
@@ -611,7 +611,7 @@ class BlockScoping {
         for (var i = 0; i < cases.length; i++) {
           var caseConsequent = cases[i].consequent[0];
           if (t.isBreakStatement(caseConsequent) && !caseConsequent.label) {
-            caseConsequent.label = this.loopLabel ||= this.file.scope.generateUidIdentifier("loop");
+            caseConsequent.label = this.loopLabel = this.loopLabel || this.file.scope.generateUidIdentifier("loop");
           }
         }
 
diff --git a/src/babel/transformation/transformers/es6/classes.js b/src/babel/transformation/transformers/es6/classes.js
index 7bb21f85b808..332b0767b384 100644
--- a/src/babel/transformation/transformers/es6/classes.js
+++ b/src/babel/transformation/transformers/es6/classes.js
@@ -558,11 +558,11 @@ class ClassTransformer {
       var body;
       var target;
       if (node.static) {
-        initializers = this.staticInitializersId ||= this.scope.generateUidIdentifier("staticInitializers");
+        initializers = this.staticInitializersId = this.staticInitializersId || this.scope.generateUidIdentifier("staticInitializers");
         body = this.staticPropBody;
         target = this.classRef;
       } else {
-        initializers = this.instanceInitializersId ||= this.scope.generateUidIdentifier("instanceInitializers");
+        initializers = this.instanceInitializersId = this.instanceInitializersId || this.scope.generateUidIdentifier("instanceInitializers");
         body = this.instancePropBody;
         target = t.thisExpression();
       }
@@ -575,7 +575,7 @@ class ClassTransformer {
         ])
       ));
     } else {
-      node.value ||= t.identifier("undefined");
+      node.value = node.value || t.identifier("undefined");
 
       if (node.static) {
         // can just be added to the static map
diff --git a/src/babel/transformation/transformers/es6/destructuring.js b/src/babel/transformation/transformers/es6/destructuring.js
index c6adc78b6c98..108eda311b73 100644
--- a/src/babel/transformation/transformers/es6/destructuring.js
+++ b/src/babel/transformation/transformers/es6/destructuring.js
@@ -200,7 +200,7 @@ export function VariableDeclaration(node, parent, scope, file) {
 
     for (i = 0; i < nodes.length; i++) {
       node = nodes[i];
-      declar ||= t.variableDeclaration(node.kind, []);
+      declar = declar || t.variableDeclaration(node.kind, []);
 
       if (!t.isVariableDeclaration(node) && declar.kind !== node.kind) {
         throw file.errorWithNode(node, messages.get("invalidParentForThisNode"));
diff --git a/src/babel/transformation/transformers/es6/object-super.js b/src/babel/transformation/transformers/es6/object-super.js
index 7a08bdb3b39d..b261520da29a 100644
--- a/src/babel/transformation/transformers/es6/object-super.js
+++ b/src/babel/transformation/transformers/es6/object-super.js
@@ -32,7 +32,7 @@ function Property(path, node, scope, getObjectRef, file) {
 
 export function ObjectExpression(node, parent, scope, file) {
   var objectRef;
-  var getObjectRef = () => objectRef ||= scope.generateUidIdentifier("obj");
+  var getObjectRef = () => objectRef = objectRef || scope.generateUidIdentifier("obj");
 
   var propPaths = this.get("properties");
   for (var i = 0; i < node.properties.length; i++) {
diff --git a/src/babel/transformation/transformers/es6/tail-call.js b/src/babel/transformation/transformers/es6/tail-call.js
index 91f6d3040b81..7e894b020068 100644
--- a/src/babel/transformation/transformers/es6/tail-call.js
+++ b/src/babel/transformation/transformers/es6/tail-call.js
@@ -101,23 +101,23 @@ class TailCallTransformer {
   }
 
   getArgumentsId() {
-    return this.argumentsId ||= this.scope.generateUidIdentifier("arguments");
+    return this.argumentsId = this.argumentsId || this.scope.generateUidIdentifier("arguments");
   }
 
   getThisId() {
-    return this.thisId ||= this.scope.generateUidIdentifier("this");
+    return this.thisId = this.thisId || this.scope.generateUidIdentifier("this");
   }
 
   getLeftId() {
-    return this.leftId ||= this.scope.generateUidIdentifier("left");
+    return this.leftId = this.leftId || this.scope.generateUidIdentifier("left");
   }
 
   getFunctionId() {
-    return this.functionId ||= this.scope.generateUidIdentifier("function");
+    return this.functionId = this.functionId || this.scope.generateUidIdentifier("function");
   }
 
   getAgainId() {
-    return this.againId ||= this.scope.generateUidIdentifier("again");
+    return this.againId = this.againId || this.scope.generateUidIdentifier("again");
   }
 
   getParams() {
diff --git a/src/babel/transformation/transformers/internal/shadow-functions.js b/src/babel/transformation/transformers/internal/shadow-functions.js
index f1c201d2d89b..4eb4e5e375fc 100644
--- a/src/babel/transformation/transformers/internal/shadow-functions.js
+++ b/src/babel/transformation/transformers/internal/shadow-functions.js
@@ -52,11 +52,11 @@ function aliasFunction(getBody, path, scope) {
 
   var state = {
     getArgumentsId() {
-      return argumentsId ||= scope.generateUidIdentifier("arguments");
+      return argumentsId = argumentsId || scope.generateUidIdentifier("arguments");
     },
 
     getThisId() {
-      return thisId ||= scope.generateUidIdentifier("this");
+      return thisId = thisId || scope.generateUidIdentifier("this");
     }
   };
 
@@ -67,7 +67,7 @@ function aliasFunction(getBody, path, scope) {
   var body;
 
   var pushDeclaration = function (id, init) {
-    body ||= getBody();
+    body = body || getBody();
     body.unshift(t.variableDeclaration("var", [
       t.variableDeclarator(id, init)
     ]));
diff --git a/src/babel/transformation/transformers/internal/validation.js b/src/babel/transformation/transformers/internal/validation.js
index c21b524294fa..8fab413fdd30 100644
--- a/src/babel/transformation/transformers/internal/validation.js
+++ b/src/babel/transformation/transformers/internal/validation.js
@@ -21,7 +21,7 @@ export function MethodDefinition(node) {
     var isConstructor = !node.computed && t.isIdentifier(node.key, { name: "constructor" });
 
     // get ["constructor"]() {}
-    isConstructor ||= t.isLiteral(node.key, { value: "constructor" });
+    isConstructor = isConstructor || t.isLiteral(node.key, { value: "constructor" });
 
     if (isConstructor) {
       throw this.errorWithNode(messages.get("classesIllegalConstructorKind"));
diff --git a/src/babel/traversal/index.js b/src/babel/traversal/index.js
index 65112715d2bd..c70f4b2c32c7 100644
--- a/src/babel/traversal/index.js
+++ b/src/babel/traversal/index.js
@@ -122,7 +122,8 @@ traverse.explode = function (obj) {
     var aliases = t.FLIPPED_ALIAS_KEYS[type];
     if (aliases) {
       for (var i = 0; i < aliases.length; i++) {
-        obj[aliases[i]] ||= fns;
+        var alias = aliases[i];
+        obj[alias] = obj[alias] || fns;
       }
     }
   }
diff --git a/src/babel/traversal/path/index.js b/src/babel/traversal/path/index.js
index b7d6ffc201ee..185ab3757a0b 100644
--- a/src/babel/traversal/path/index.js
+++ b/src/babel/traversal/path/index.js
@@ -47,7 +47,7 @@ export default class TraversalPath {
 
   static get(parentPath: TraversalPath, context?: TraversalContext, parent, container, key, file?: File) {
     var targetNode = container[key];
-    var paths = container._paths ||= [];
+    var paths = container._paths = container._paths || [];
     var path;
 
     for (var i = 0; i < paths.length; i++) {
@@ -613,7 +613,7 @@ export default class TraversalPath {
         var match = prop.isnt("computed") && key.isIdentifier({ name: targetName });
 
         // { "foo": "obj" } or { ["foo"]: "obj" }
-        match ||= key.isLiteral({ value: targetName });
+        match = match || key.isLiteral({ value: targetName });
 
         if (match) return prop.get("value");
       }
diff --git a/src/babel/traversal/scope.js b/src/babel/traversal/scope.js
index a8e89724facc..2a844429222d 100644
--- a/src/babel/traversal/scope.js
+++ b/src/babel/traversal/scope.js
@@ -242,8 +242,8 @@ export default class Scope {
     if (kind === "hoisted" && local.kind === "let") return;
 
     var duplicate = false;
-    duplicate ||= kind === "let" || kind === "const" || local.kind === "let" || local.kind === "const" || local.kind === "module";
-    duplicate ||= local.kind === "param" && (kind === "let" || kind === "const");
+    if (!duplicate) duplicate = kind === "let" || kind === "const" || local.kind === "let" || local.kind === "const" || local.kind === "module";
+    if (!duplicate) duplicate = local.kind === "param" && (kind === "let" || kind === "const");
 
     if (duplicate) {
       throw this.file.errorWithNode(id, messages.get("scopeDuplicateDeclaration", name), TypeError);
@@ -255,7 +255,7 @@ export default class Scope {
    */
 
   rename(oldName: string, newName: string, block?) {
-    newName ||= this.generateUidIdentifier(oldName).name;
+    newName = newName || this.generateUidIdentifier(oldName).name;
 
     var info = this.getBinding(oldName);
     if (!info) return;
diff --git a/src/babel/types/index.js b/src/babel/types/index.js
index e731f40aded8..3b31d8868886 100644
--- a/src/babel/types/index.js
+++ b/src/babel/types/index.js
@@ -17,7 +17,7 @@ function registerType(type: string, skipAliasCheck?: boolean) {
   };
 
   t[`assert${type}`] = function (node, opts) {
-    opts ||= {};
+    opts = opts || {};
     if (!is(node, opts)) {
       throw new Error(`Expected type ${JSON.stringify(type)} with option ${JSON.stringify(opts)}`);
     }
@@ -42,7 +42,7 @@ each(t.VISITOR_KEYS, function (keys, type) {
 
 each(t.ALIAS_KEYS, function (aliases, type) {
   each(aliases, function (alias) {
-    var types = t.FLIPPED_ALIAS_KEYS[alias] ||= [];
+    var types = t.FLIPPED_ALIAS_KEYS[alias] = t.FLIPPED_ALIAS_KEYS[alias] || [];
     types.push(type);
   });
 });