Add 'breakpoint' similar to 'debugger' from JavaScript.

dart/runtime/lib/corelib_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.
 
 # Sources visible via default library.
 
 {
   'sources': [
     'core_patch.dart',
     # The above file needs to be first as it imports required libraries.
     'bool.cc',
     'bool_patch.dart',
+    'breakpoint_patch.dart',
     'date.cc',
     'date_patch.dart',
     'array.cc',
     'array.dart',
     'array_patch.dart',
     'double.cc',
     'double.dart',
     'double_patch.dart',
     'errors.cc',
     'errors_patch.dart',
@@ skipping 26 matching lines @@
     'string.cc',
     'string_patch.dart',
     'string_buffer_patch.dart',
     'type_patch.dart',
     'uri.cc',
     'uri_patch.dart',
     'weak_property.dart',
     'weak_property.cc',
   ],
 }