Dart: Removes all but native calls and the handle watcher from the snapshot.

mojo/dart/test/async_await_test.dart

Index: mojo/dart/test/async_await_test.dart
diff --git a/mojo/dart/test/async_await_test.dart b/mojo/dart/test/async_await_test.dart
index bd38bcb4f968a2e24c70a906959fb26825b44f47..ef1f4663caa54fae1c8f5db7947beaaf01ab4a94 100644
--- a/mojo/dart/test/async_await_test.dart
+++ b/mojo/dart/test/async_await_test.dart
@@ -3,7 +3,8 @@
 // found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:mojo.core' as core;
+
+import 'package:mojo/public/dart/core.dart';
 
 main() async {
   var x = await (new Future.value(42));