Avoid storing Symbol property on React object in ApolloContext.ts.

Fixes the Symbol-related errors reported in #7370.

src/react/context/ApolloContext.ts

@@ -1,5 +1,6 @@
-import React from 'react';
+import * as React from 'react';
 import { ApolloClient } from '../../core';
+import { canUseWeakMap } from '../../utilities';
 
 export interface ApolloContextValue {
   client?: ApolloClient<object>;
@@ -10,27 +11,21 @@ export interface ApolloContextValue {
 // (which can lead to problems like having an Apollo Client instance added
 // in one context, then attempting to retrieve it from another different
 // context), a single Apollo context is created and tracked in global state.
-// Since the created context is React specific, we've decided to attach it to
-// the `React` object for sharing.
+// We use React.createContext as the key instead of just React to avoid
+// ambiguities between default and namespace React imports.
 
-// If Symbol's aren't available, we'll use a fallback string as the context
-// property (we're looking at you, IE11).
-const contextSymbol = typeof Symbol === 'function' && Symbol.for ?
-  Symbol.for('__APOLLO_CONTEXT__') :
-  '__APOLLO_CONTEXT__';
-
-export function resetApolloContext() {
-  Object.defineProperty(React, contextSymbol, {
-    value: React.createContext<ApolloContextValue>({}),
-    enumerable: false,
-    configurable: true,
-    writable: false,
-  });
-}
+const cache = new (canUseWeakMap ? WeakMap : Map)<
+  typeof React.createContext,
+  React.Context<ApolloContextValue>
+>();
 
 export function getApolloContext() {
-  if (!(React as any)[contextSymbol]) {
-    resetApolloContext();
+  let context = cache.get(React.createContext)!;
+  if (!context) {
+    context = React.createContext<ApolloContextValue>({});
+    cache.set(React.createContext, context);
   }
-  return (React as any)[contextSymbol] as React.Context<ApolloContextValue>;
+  return context;
 }
+
+export { getApolloContext as resetApolloContext }