Add pass to extract ES6 class definitions out of non-simple contexts.

Currently, JSCompiler only transpiles ES6 classes that appear as declarations or simple assignments in the original source code. Classes in other contexts are rejected as compilation errors.

There's no deep reason for this restriction. JSCompiler should accept ES6 classes anywhere they can appear according to the grammar of the language.

This CL adds a pass that runs before the main Es6ToEs3Converter pass. It replaces class definitions in complex contexts:

```
f(class{ /* ... */});
```

with an immediately preceding alias:
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=124866181

src/com/google/javascript/jscomp/ES6ModuleLoader.java

@@ -188,21 +188,28 @@ static boolean isAbsoluteIdentifier(String name) {
     return name.startsWith(MODULE_SLASH);
   }
 
+  /**
+   * Turns a filename into a JS identifier that can be used in rewritten code.
+   * Removes leading ./, replaces / with $, removes trailing .js
+   * and replaces - with _.
+   */
+  public static String toJSIdentifier(URI filename) {
+    return stripJsExtension(filename.toString())
+        .replaceAll("^\\." + Pattern.quote(MODULE_SLASH), "")
+        .replace(MODULE_SLASH, "$")
+        .replace('\\', '$')
+        .replace('-', '_')
+        .replace(':', '_')
+        .replace('.', '_')
+        .replace("%20", "_");
+  }
+
   /**
    * Turns a filename into a JS identifier that is used for moduleNames in
    * rewritten code. Removes leading ./, replaces / with $, removes trailing .js
    * and replaces - with _. All moduleNames get a "module$" prefix.
    */
   public static String toModuleName(URI filename) {
-    String moduleName =
-        stripJsExtension(filename.toString())
-            .replaceAll("^\\." + Pattern.quote(MODULE_SLASH), "")
-            .replace(MODULE_SLASH, "$")
-            .replace('\\', '$')
-            .replace('-', '_')
-            .replace(':', '_')
-            .replace('.', '_')
-            .replace("%20", "_");
-    return "module$" + moduleName;
+    return "module$" + toJSIdentifier(filename);
   }
 }

src/com/google/javascript/jscomp/Es6ExtractClasses.java

@@ -0,0 +1,104 @@
+/*
+ * Copyright 2016 The Closure Compiler Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.javascript.jscomp;
+
+import com.google.javascript.jscomp.ExpressionDecomposer.DecompositionType;
+import com.google.javascript.rhino.IR;
+import com.google.javascript.rhino.Node;
+import com.google.javascript.rhino.Token;
+
+import java.util.HashSet;
+import java.util.Set;
+
+/**
+ * Extracts ES6 classes defined in function calls to local constants.
+ * <p>
+ * Example:
+ * Before: <code>foo(class { constructor() {} });</code>
+ * After:
+ * <code>
+ *   const $jscomp$classdecl$var0 = class { constructor() {} };
+ *   foo($jscomp$classdecl$var0);
+ * </code>
+ * <p>
+ * This must be done before {@link Es6ToEs3Converter}, because that pass only handles classes
+ * that are declarations or simple assignments.
+ * @see Es6ToEs3Converter#visitClass(Node, Node)
+ */
+public final class Es6ExtractClasses
+    extends NodeTraversal.AbstractPostOrderCallback implements HotSwapCompilerPass {
+
+  private static final String CLASS_DECL_VAR = "$classdecl$var";
+
+  private final AbstractCompiler compiler;
+  private final ExpressionDecomposer expressionDecomposer;
+  private int classDeclVarCounter = 0;
+
+  Es6ExtractClasses(AbstractCompiler compiler) {
+    this.compiler = compiler;
+    Set<String> consts = new HashSet<>();
+    this.expressionDecomposer = new ExpressionDecomposer(
+        compiler,
+        compiler.getUniqueNameIdSupplier(),
+        consts,
+        Scope.createGlobalScope(new Node(Token.SCRIPT)));
+  }
+
+  @Override
+  public void process(Node externs, Node root) {
+    NodeTraversal.traverseRootsEs6(compiler, this, externs, root);
+  }
+
+  @Override
+  public void hotSwapScript(Node scriptRoot, Node originalRoot) {
+    NodeTraversal.traverseEs6(compiler, scriptRoot, this);
+  }
+
+  @Override
+  public void visit(NodeTraversal t, Node n, Node parent) {
+    if (n.isClass() && shouldExtractClass(n, parent)) {
+      extractClass(n, parent);
+    }
+  }
+
+  private boolean shouldExtractClass(Node classNode, Node parent) {
+    if (NodeUtil.isClassDeclaration(classNode)
+        || parent.isName()
+        || (parent.isAssign() && parent.getParent().isExprResult())) {
+      // No need to extract. Handled directly by Es6ToEs3Converter.ClassDeclarationMetadata#create.
+      return false;
+    }
+    // Don't extract the class if it's not safe to do so. For example,
+    // var c = maybeTrue() && class extends someSideEffect() {};
+    // TODO(brndn): it is possible to be less conservative. If the classNode is DECOMPOSABLE,
+    // we could use the expression decomposer to move it out of the way.
+    return expressionDecomposer.canExposeExpression(classNode) == DecompositionType.MOVABLE;
+  }
+
+  private void extractClass(Node classNode, Node parent) {
+    String name = ES6ModuleLoader.toJSIdentifier(
+        ES6ModuleLoader.createUri(classNode.getStaticSourceFile().getName()))
+        + CLASS_DECL_VAR
+        + (classDeclVarCounter++);
+    Node statement = NodeUtil.getEnclosingStatement(parent);
+    parent.replaceChild(classNode, IR.name(name));
+    Node classDeclaration = IR.constNode(IR.name(name), classNode)
+        .useSourceInfoIfMissingFromForTree(classNode);
+    statement.getParent().addChildBefore(classDeclaration, statement);
+    compiler.reportCodeChange();
+  }
+}

src/com/google/javascript/jscomp/ExpressionDecomposer.java

@@ -672,6 +672,8 @@ static Node findExpressionRoot(Node subExpression) {
         case RETURN:
         case THROW:
         case VAR:
+        case CONST:
+        case LET:
           Preconditions.checkState(child == parent.getFirstChild());
           return parent;
         // Any of these indicate an unsupported expression:

src/com/google/javascript/jscomp/TranspilationPasses.java

@@ -50,12 +50,22 @@ public static void addEs6EarlyPasses(List<PassFactory> passes) {
    */
   public static void addEs6LatePasses(List<PassFactory> passes) {
     passes.add(es6ConvertSuper);
+    passes.add(es6ExtractClasses);
     passes.add(convertEs6ToEs3);
     passes.add(rewriteBlockScopedDeclaration);
     passes.add(rewriteGenerators);
     passes.add(rewritePolyfills);
   }
 
+
+  static final HotSwapPassFactory es6ExtractClasses =
+      new HotSwapPassFactory("Es6ExtractClasses", true) {
+        @Override
+        protected HotSwapCompilerPass create(AbstractCompiler compiler) {
+          return new Es6ExtractClasses(compiler);
+        }
+      };
+
   static final HotSwapPassFactory es6RewriteDestructuring =
       new HotSwapPassFactory("Es6RewriteDestructuring", true) {
         @Override

test/com/google/javascript/jscomp/Es6ExtractClassesTest.java

@@ -0,0 +1,89 @@
+/*
+ * Copyright 2016 The Closure Compiler Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.javascript.jscomp;
+
+import com.google.javascript.jscomp.CompilerOptions.LanguageMode;
+
+public final class Es6ExtractClassesTest extends CompilerTestCase {
+
+  @Override
+  protected CompilerPass getProcessor(Compiler compiler) {
+    return new Es6ExtractClasses(compiler);
+  }
+
+  @Override
+  protected void setUp() {
+    setAcceptedLanguage(LanguageMode.ECMASCRIPT6);
+    disableTypeCheck();
+    runTypeCheckAfterProcessing = true;
+  }
+
+  public void testExtractionFromCall() {
+    test(
+        "f(class{});",
+        LINE_JOINER.join(
+            "const testcode$classdecl$var0 = class {};", "f(testcode$classdecl$var0);"));
+  }
+
+  public void testConstAssignment() {
+    test(
+        "var foo = bar(class {});",
+        LINE_JOINER.join(
+            "const testcode$classdecl$var0 = class {};",
+            "var foo = bar(testcode$classdecl$var0);"));
+  }
+
+  public void testLetAssignment() {
+    test(
+        "let foo = bar(class {});",
+        LINE_JOINER.join(
+            "const testcode$classdecl$var0 = class {};",
+            "let foo = bar(testcode$classdecl$var0);"));
+  }
+
+  public void testVarAssignment() {
+    test(
+        "var foo = bar(class {});",
+        LINE_JOINER.join(
+            "const testcode$classdecl$var0 = class {};",
+            "var foo = bar(testcode$classdecl$var0);"));
+  }
+
+  public void testConditionalBlocksExtractionFromCall() {
+    testSame("maybeTrue() && f(class{});");
+  }
+
+  public void testExtractionFromArrayLiteral() {
+    test(
+        "var c = [class C {}];",
+        LINE_JOINER.join(
+            "const testcode$classdecl$var0 = class C {};", "var c = [testcode$classdecl$var0];"));
+  }
+
+  public void testConditionalBlocksExtractionFromArrayLiteral() {
+    testSame("var c = maybeTrue() && [class {}]");
+  }
+
+  public void testTernaryOperatorBlocksExtraction() {
+    testSame("var c = maybeTrue() ? class A {} : class B {}");
+  }
+
+  public void testClassesHandledByEs6ToEs3Converter() {
+    testSame("class C{}");
+    testSame("var c = class {};");
+  }
+}