Make the check for ZoneAwarePromise more stringent

`resolvePromise` assumes that if a value is an `instanceof` ZoneAwarePromise
then it has the properties "__zone_symbol__state" and "__zone_symbol__value"
and it _is_ a true ZoneAwarePromise; however, a user can construct a value that
breaks this assumption by inheriting from ZoneAwarePromise without actually
having those properties or being a true ZoneAwarePromise (for example, by
attempting to subclass Promise).

We can fix this by adding checks for "__zone_symbol__state" and
"__zone_symbol__value" to `resolvePromise`.

lib/zone.ts

@@ -1030,7 +1030,10 @@ const Zone: ZoneType = (function(global: any) {
   function resolvePromise(
       promise: ZoneAwarePromise<any>, state: boolean, value: any): ZoneAwarePromise<any> {
     if (promise[symbolState] === UNRESOLVED) {
-      if (value instanceof ZoneAwarePromise && value[symbolState] !== UNRESOLVED) {
+      if (value instanceof ZoneAwarePromise &&
+          value.hasOwnProperty(symbolState) &&
+          value.hasOwnProperty(symbolValue) &&
+          value[symbolState] !== UNRESOLVED) {
         clearRejectedNoCatch(<Promise<any>>value);
         resolvePromise(promise, value[symbolState], value[symbolValue]);
       } else if (isThenable(value)) {

test/common/Promise.spec.ts

@@ -331,6 +331,53 @@ describe(
         });
       });
 
+      describe('Promise subclasses', function() {
+        function MyPromise(init) {
+          this._promise = new Promise(init);
+        }
+
+        MyPromise.prototype.catch = function _catch() {
+          return this._promise.catch.apply(this._promise, arguments);
+        };
+
+        MyPromise.prototype.then = function then() {
+          return this._promise.then.apply(this._promise, arguments);
+        };
+
+        var setPrototypeOf = (Object as any).setPrototypeOf || function(obj, proto) {
+          obj.__proto__ = proto;
+          return obj;
+        }
+
+        setPrototypeOf(MyPromise.prototype, Promise.prototype);
+
+        it('should reject if the Promise subclass rejects', function() {
+          var myPromise = new MyPromise(function(resolve, reject) {
+            reject('foo');
+          });
+
+          return Promise.resolve().then(function() {
+            return myPromise;
+          }).then(function() {
+            throw new Error('Unexpected resolution');
+          }, function(result) {
+            expect(result).toBe('foo');
+          });
+        });
+
+        it('should resolve if the Promise subclass resolves', function() {
+          var myPromise = new MyPromise(function(resolve, reject) {
+            resolve('foo');
+          });
+
+          return Promise.resolve().then(function() {
+            return myPromise;
+          }).then(function(result) {
+            expect(result).toBe('foo');
+          });
+        });
+      });
+
       describe('fetch', ifEnvSupports('fetch', function() {
                  it('should work for text response', function(done) {
                    testZone.run(function() {