Add stubs for Timer.tick, ready for Dart 2.0 core lib changes.

#Dart2

Tested: https://tap.corp.google.com/ui#id=OCL:180837836:BASE:180879972:1515123567313:29de349d
PiperOrigin-RevId: 181173154

angular/lib/src/core/zone/ng_zone.dart

@@ -397,6 +397,14 @@ class WrappedTimer implements Timer {
   }
 
   bool get isActive => _timer.isActive;
+
+  @override
+  // TODO: Dart 2.0 requires this method to be implemented.
+  // See https://github.com/dart-lang/sdk/issues/31664
+  // ignore: override_on_non_overriding_getter
+  int get tick {
+    throw new UnimplementedError("tick");
+  }
 }
 
 /// Stores error information; delivered via [NgZone.onError] stream.