Move StopwatchImplementation from coreimpl to core as _StopwatchImpl.

runtime/lib/lib_impl_sources.gypi

 # Copyright (c) 2011, 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.
 
 # Implementation sources.
 
 {
   'sources': [
     'array.cc',
     'array.dart',
     'array_patch.dart',
     'growable_array.cc',
     'growable_array.dart',
     'immutable_map.dart',
     'literal_factory.dart',
     'math.dart',
     'math.cc',
     'object.cc',
     'regexp.cc',
     'regexp_jsc.cc',
     'regexp_jsc.h',
     'regexp_patch.dart',
-    'stopwatch_patch.dart',
     'string.cc',
     'string_base.dart',
     'string_patch.dart',
   ],
 }