Strong mode update for swarm

TBR=sigmund@google.com

Change-Id: Id53d58d05e27bffe17c810614dd37294acc3491d
Reviewed-on: https://dart-review.googlesource.com/53242
Reviewed-by: Stephen Adams <sra@google.com>
Commit-Queue: Stephen Adams <sra@google.com>

samples-dev/swarm/swarm_ui_lib/touch/TouchUtil.dart

@@ -359,4 +359,12 @@ class MockTouchEvent implements TouchEvent {
   /*InputDevice*/ get sourceDevice {
     throw new UnimplementedError();
   }
+
+  bool get composed {
+    throw new UnimplementedError();
+  }
+
+  List<EventTarget> composedPath() {
+    throw new UnimplementedError();
+  }
 }