Cache query documents transformed by InMemoryCache.

After #3444 removed `Map`-based caching for `addTypenameToDocument` (in
order to fix memory leaks), the `InMemoryCache#transformDocument` method
now creates a completely new `DocumentNode` every time it's called
(assuming this.addTypename is true, which it is by default).

This commit uses a `WeakMap` to cache calls to `addTypenameToDocument` in
`InMemoryCache#transformDocument`, so that repeated cache reads will no
longer create an unbounded number of new `DocumentNode` objects. The
benefit of the `WeakMap` is that it does not prevent its keys (the
original `DocumentNode` objects) from being garbage collected, which is
another way of preventing memory leaks.  Note that `WeakMap` may have to
be polyfilled in older browsers, but there are many options for that.

This optimization will be important for #3394, since the query document is
involved in cache keys used to store cache partial query results.

cc @hwillson @jbaxleyiii @brunorzn

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

@@ -43,6 +43,7 @@ export class InMemoryCache extends ApolloCache<NormalizedCacheObject> {
   protected optimistic: OptimisticStoreItem[] = [];
   private watches: Cache.WatchOptions[] = [];
   private addTypename: boolean;
+  private typenameDocumentCache = new WeakMap<DocumentNode, DocumentNode>();
 
   // Set this while in a transaction to prevent broadcasts...
   // don't forget to turn it back on!
@@ -204,7 +205,16 @@ export class InMemoryCache extends ApolloCache<NormalizedCacheObject> {
   }
 
   public transformDocument(document: DocumentNode): DocumentNode {
-    if (this.addTypename) return addTypenameToDocument(document);
+    if (this.addTypename) {
+      let result = this.typenameDocumentCache.get(document);
+      if (!result) {
+        this.typenameDocumentCache.set(
+          document,
+          (result = addTypenameToDocument(document)),
+        );
+      }
+      return result;
+    }
     return document;
   }