fix(promise): allow Promise subclassing

When loading es6-shim the es6-shim clovers Zone Promise because it does not allow subclassing.
This change now supports subclassing and verifies that constructor throws if invoked on non-Promise subclass.

This change now allows es6-shim to be loaded after Zone.js

lib/zone.ts

@@ -1060,6 +1060,9 @@ const Zone: ZoneType = (function(global: any) {
     constructor(executor: (resolve : (value?: R | Thenable<R>) => void,
                            reject: (error?: any) => void) => void) {
       const promise: ZoneAwarePromise<R> = this;
+      if (!(promise instanceof ZoneAwarePromise)) {
+        throw new Error('Must be an instanceof Promise.');
+      }
       promise[symbolState] = UNRESOLVED;
       promise[symbolValue] = []; // queue;
       try {
@@ -1072,7 +1075,7 @@ const Zone: ZoneType = (function(global: any) {
     then<R, U>(onFulfilled?: (value: R) => U | Thenable<U>,
                onRejected?: (error: any) => U | Thenable<U>): Promise<R>
     {
-      const chainPromise: Promise<R> = new ZoneAwarePromise(null);
+      const chainPromise: Promise<R> = new (this.constructor as typeof ZoneAwarePromise)(null);
       const zone = Zone.current;
       if (this[symbolState] == UNRESOLVED ) {
         (<any[]>this[symbolValue]).push(zone, chainPromise, onFulfilled, onRejected);

test/common/Promise.spec.ts

@@ -45,7 +45,22 @@ describe('Promise', ifEnvSupports('Promise', function () {
   it ('should make sure that new Promise is instance of Promise', () => {
     expect(Promise.resolve(123) instanceof Promise).toBe(true);
     expect(new Promise(() => null) instanceof Promise).toBe(true);
-  })
+  });
+
+  it('should ensure that Promise this is instanceof Promise', () => {
+      expect(() => {
+        Promise.call({}, null);
+      }).toThrowError('Must be an instanceof Promise.');
+  });
+
+  it('should allow subclassing', () => {
+    class MyPromise extends Promise<any> {
+      constructor(fn: any) {
+        super(fn);
+      }
+    }
+    expect(new MyPromise(null).then(() => null) instanceof MyPromise).toBe(true);
+  });
 
   it('should intercept scheduling of resolution and then', (done) => {
     pZone.run(() => {