diff --git a/.babelrc.js b/.babelrc.js
index 387d74220c..209874aab7 100644
--- a/.babelrc.js
+++ b/.babelrc.js
@@ -12,7 +12,6 @@ module.exports = {
   }]],
   "plugins": [
     "./resources/inline-invariant",
-    "@babel/plugin-proposal-class-properties",
     "@babel/plugin-transform-flow-strip-types",
     ["@babel/plugin-transform-classes", {"loose": true}],
     ["@babel/plugin-transform-destructuring", {"loose": true}],
diff --git a/package.json b/package.json
index b60378c6b6..93e0de2ba0 100644
--- a/package.json
+++ b/package.json
@@ -51,7 +51,6 @@
   "devDependencies": {
     "@babel/cli": "7.2.0",
     "@babel/core": "7.2.2",
-    "@babel/plugin-proposal-class-properties": "7.2.1",
     "@babel/plugin-transform-flow-strip-types": "7.2.0",
     "@babel/polyfill": "7.0.0",
     "@babel/preset-env": "7.2.0",
diff --git a/src/type/schema.js b/src/type/schema.js
index 439a6a5543..9f55442e63 100644
--- a/src/type/schema.js
+++ b/src/type/schema.js
@@ -93,7 +93,7 @@ export class GraphQLSchema {
   _directives: $ReadOnlyArray<GraphQLDirective>;
   _typeMap: TypeMap;
   _implementations: ObjMap<Array<GraphQLObjectType>>;
-  _possibleTypeMap: ?ObjMap<ObjMap<boolean>>;
+  _possibleTypeMap: ObjMap<ObjMap<boolean>>;
   // Used as a cache for validateSchema().
   __validationErrors: ?$ReadOnlyArray<GraphQLError>;
   // Referenced by validateSchema().
@@ -105,6 +105,8 @@ export class GraphQLSchema {
     if (config && config.assumeValid) {
       this.__validationErrors = [];
     } else {
+      this.__validationErrors = undefined;
+
       // Otherwise check for common mistakes during construction to produce
       // clear and early error messages.
       invariant(
@@ -161,6 +163,8 @@ export class GraphQLSchema {
     // Storing the resulting map for reference by the schema.
     this._typeMap = typeMap;
 
+    this._possibleTypeMap = Object.create(null);
+
     // Keep track of all implementations by interface name.
     this._implementations = Object.create(null);
     for (const typeName of Object.keys(this._typeMap)) {
@@ -215,10 +219,7 @@ export class GraphQLSchema {
     abstractType: GraphQLAbstractType,
     possibleType: GraphQLObjectType,
   ): boolean {
-    let possibleTypeMap = this._possibleTypeMap;
-    if (!possibleTypeMap) {
-      this._possibleTypeMap = possibleTypeMap = Object.create(null);
-    }
+    const possibleTypeMap = this._possibleTypeMap;
 
     if (!possibleTypeMap[abstractType.name]) {
       const possibleTypes = this.getPossibleTypes(abstractType);
diff --git a/src/validation/ValidationContext.js b/src/validation/ValidationContext.js
index e6c924c124..170cb36787 100644
--- a/src/validation/ValidationContext.js
+++ b/src/validation/ValidationContext.js
@@ -46,7 +46,7 @@ type VariableUsage = {|
 export class ASTValidationContext {
   _ast: DocumentNode;
   _errors: Array<GraphQLError>;
-  _fragments: ObjMap<FragmentDefinitionNode>;
+  _fragments: ?ObjMap<FragmentDefinitionNode>;
   _fragmentSpreads: Map<SelectionSetNode, $ReadOnlyArray<FragmentSpreadNode>>;
   _recursivelyReferencedFragments: Map<
     OperationDefinitionNode,
@@ -56,6 +56,7 @@ export class ASTValidationContext {
   constructor(ast: DocumentNode): void {
     this._ast = ast;
     this._errors = [];
+    this._fragments = undefined;
     this._fragmentSpreads = new Map();
     this._recursivelyReferencedFragments = new Map();
   }
diff --git a/yarn.lock b/yarn.lock
index 9b6229e1d3..0ccd48e364 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -81,17 +81,6 @@
     "@babel/traverse" "^7.1.0"
     "@babel/types" "^7.0.0"
 
-"@babel/helper-create-class-features-plugin@^7.2.1":
-  version "7.2.2"
-  resolved "https://registry.yarnpkg.com/@babel/helper-create-class-features-plugin/-/helper-create-class-features-plugin-7.2.2.tgz#aac79552e41c94716a804d371e943f20171df0c5"
-  integrity sha512-Q4qZE5wS3NWpOS6UV9yhIS/NmSyf2keF0E0IwDvx8WxTheVopVIY6BSQ/0vz72OTTruz0cOA2yIUh6Kdg3qprA==
-  dependencies:
-    "@babel/helper-function-name" "^7.1.0"
-    "@babel/helper-member-expression-to-functions" "^7.0.0"
-    "@babel/helper-optimise-call-expression" "^7.0.0"
-    "@babel/helper-plugin-utils" "^7.0.0"
-    "@babel/helper-replace-supers" "^7.1.0"
-
 "@babel/helper-define-map@^7.1.0":
   version "7.1.0"
   resolved "https://registry.yarnpkg.com/@babel/helper-define-map/-/helper-define-map-7.1.0.tgz#3b74caec329b3c80c116290887c0dd9ae468c20c"
@@ -255,14 +244,6 @@
     "@babel/helper-remap-async-to-generator" "^7.1.0"
     "@babel/plugin-syntax-async-generators" "^7.2.0"
 
-"@babel/plugin-proposal-class-properties@7.2.1":
-  version "7.2.1"
-  resolved "https://registry.yarnpkg.com/@babel/plugin-proposal-class-properties/-/plugin-proposal-class-properties-7.2.1.tgz#c734a53e0a1ec40fe5c22ee5069d26da3b187d05"
-  integrity sha512-/4FKFChkQ2Jgb8lBDsvFX496YTi7UWTetVgS8oJUpX1e/DlaoeEK57At27ug8Hu2zI2g8bzkJ+8k9qrHZRPGPA==
-  dependencies:
-    "@babel/helper-create-class-features-plugin" "^7.2.1"
-    "@babel/helper-plugin-utils" "^7.0.0"
-
 "@babel/plugin-proposal-json-strings@^7.2.0":
   version "7.2.0"
   resolved "https://registry.yarnpkg.com/@babel/plugin-proposal-json-strings/-/plugin-proposal-json-strings-7.2.0.tgz#568ecc446c6148ae6b267f02551130891e29f317"