Move core and coreimpl from corelib/ to lib/core and lib/coreimpl.

lib/coreimpl/stopwatch_implementation.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * A simple implementation of the [Stopwatch] interface.
  */
 class StopwatchImplementation implements Stopwatch {
   // The _start and _stop fields capture the time when [start] and [stop]
   // are called respectively.
@@ skipping 55 matching lines @@
 
   int elapsedInMs() {
     return (elapsed() * 1000) ~/ frequency();
   }
 
   int frequency() => _frequency();
 
   external static int _frequency();
   external static int _now();
 }