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

dart/sdk/lib/_internal/lib/js_primitives.dart

 // Copyright (c) 2013, 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.
 
 /// dart2js "primitives", that is, features that cannot be implemented without
 /// access to JavaScript features.
 library dart2js._js_primitives;
 
 import 'dart:_foreign_helper' show
     JS;
@@ skipping 30 matching lines @@
   if (JS('bool', r'typeof print == "function"')) {
     JS('void', r'print(#)', string);
     return;
   }
 
   // This is somewhat nasty, but we don't want to drag in a bunch of
   // dependencies to handle a situation that cannot happen. So we
   // avoid using Dart [:throw:] and Dart [toString].
   JS('void', 'throw "Unable to print message: " + String(#)', string);
 }
+
+void breakpoint() {
+  JS('void', 'debugger');
+}