Polyfill correct Map#set and Set#add return behavior.

Implementing my own suggestion:
apollographql#4012 (review)

This change would be sufficient to fix apollographql#4024, though apollographql#4012 helps as well.

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

@@ -1,6 +1,23 @@
-const frozen = {};
-const frozenTestMap = new Map();
+// Make sure Map.prototype.set returns the Map instance, per spec.
+// https://github.com/apollographql/apollo-client/issues/4024
+const testMap = new Map();
+if (testMap.set(1, 2) !== testMap) {
+  const { set } = testMap;
+  Map.prototype.set = function (...args) {
+    return set.apply(this, args) || this;
+  };
+}
 
+// Make sure Set.prototype.add returns the Set instance, per spec.
+const testSet = new Set();
+if (testSet.add(3) !== testSet) {
+  const { add } = testSet;
+  Set.prototype.add = function (...args) {
+    return add.apply(this, args) || this;
+  };
+}
+
+const frozen = {};
 if (typeof Object.freeze === 'function') {
   Object.freeze(frozen);
 }
@@ -12,13 +29,13 @@ try {
   // objects before they become non-extensible:
   // https://github.com/facebook/react-native/blob/98a6f19d7c/Libraries/vendor/core/Map.js#L44-L50
   // https://github.com/apollographql/react-apollo/issues/2442#issuecomment-426489517
-  frozenTestMap.set(frozen, frozen).delete(frozen);
+  testMap.set(frozen, frozen).delete(frozen);
 } catch {
   const wrap = (method: <T>(obj: T) => T): typeof method => {
     return method && (obj => {
       try {
         // If .set succeeds, also call .delete to avoid leaking memory.
-        frozenTestMap.set(obj, obj).delete(obj);
+        testMap.set(obj, obj).delete(obj);
       } finally {
         // If .set or .delete fails, the exception will be silently swallowed
         // by this return-from-finally statement: