feat(test): added not.toBeNull

Author: vsavkin

modules/angular2/src/test_lib/test_lib.dart

@@ -114,6 +114,7 @@ class NotExpect extends gns.NotExpect {
 
  void toEqual(expected) => toHaveSameProps(expected);
  void toBePromise() => _expect(actual is Future, equals(false));
+ void toBeNull() => _expect(actual == null, equals(false));
  Function get _expect => gns.guinness.matchers.expect;
 }