Change NotImplementedException to UnimplementedError.

samples/ui_lib/touch/TouchUtil.dart

Index: samples/ui_lib/touch/TouchUtil.dart
diff --git a/samples/ui_lib/touch/TouchUtil.dart b/samples/ui_lib/touch/TouchUtil.dart
index fc0a997c0bab0f4f843488ba5a7f513f574cd037..5c88f0d581ad4573e5ecf0d4eea7ed6aebd35b65 100644
--- a/samples/ui_lib/touch/TouchUtil.dart
+++ b/samples/ui_lib/touch/TouchUtil.dart
@@ -166,10 +166,10 @@ class MockTouch implements Touch {
 
   EventTarget get target => wrapped.target;
 
-  num get webkitForce { throw new NotImplementedException(); }
-  int get webkitRadiusX { throw new NotImplementedException(); }
-  int get webkitRadiusY { throw new NotImplementedException(); }
-  num get webkitRotationAngle { throw new NotImplementedException(); }
+  num get webkitForce { throw new UnimplementedError(); }
+  int get webkitRadiusX { throw new UnimplementedError(); }
+  int get webkitRadiusY { throw new UnimplementedError(); }
+  num get webkitRotationAngle { throw new UnimplementedError(); }
 }
 
 class MockTouchEvent implements TouchEvent {