Avoid using prototype objects as keys in QueryKeyMaker Maps.

This temporarily mitigates the prototype chain bug that will be fixed with
this React Native PR that I submitted yesterday:
facebook/react-native#21492

I would like to find a way to bundle a different Map polyfill in React
Native apps, without increasing bundle sizes for non-RN apps, but that has
proven tricky so far. For future reference, this seems to be the way to do
it (thanks to @peggyrayzis for the tip):
https://facebook.github.io/react-native/docs/platform-specific-code#platform-specific-extensions

Until a new version of React Native is released with these changes
included, the simplest way to fix the problem is simply to avoid storing
any prototype objects in a Map, so that's what I've done in this commit.

We are already wrapping Object.freeze and friends in src/fixPolyfills.ts,
which should make it possible to use frozen objects as Map keys (the other
bug that I addressed in the React Native PR linked above).

packages/apollo-cache-inmemory/src/queryKeyMaker.ts

@@ -3,6 +3,7 @@ import { DocumentNode, SelectionSetNode, FragmentSpreadNode, FragmentDefinitionN
 import { QueryDocumentKeys } from "graphql/language/visitor";
 
 const CIRCULAR = Object.create(null);
+const objToStr = Object.prototype.toString;
 
 export class QueryKeyMaker {
   private perQueryKeyMakers = new Map<DocumentNode, PerQueryKeyMaker>();
@@ -99,7 +100,7 @@ class PerQueryKeyMaker {
   private lookupArray(array: any[]): object {
     const elements = array.map(this.lookupAny, this);
     return this.cacheKeyRoot.lookup(
-      Array.prototype,
+      objToStr.call(array),
       this.cacheKeyRoot.lookupArray(elements),
     );
   }
@@ -108,7 +109,7 @@ class PerQueryKeyMaker {
     const keys = safeSortedKeys(object);
     const values = keys.map(key => this.lookupAny(object[key]));
     return this.cacheKeyRoot.lookup(
-      Object.getPrototypeOf(object),
+      objToStr.call(object),
       this.cacheKeyRoot.lookupArray(keys),
       this.cacheKeyRoot.lookupArray(values),
     );