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

dart/sdk/lib/_internal/lib/core_patch.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.
 
 // Patch file for dart:core classes.
 import "dart:_internal" as _symbol_dev;
 import 'dart:_interceptors';
 import 'dart:_js_helper' show checkNull,
                               getRuntimeType,
                               JSSyntaxRegExp,
                               Primitives,
                               stringJoinUnchecked,
                               objectHashCode;
 
+import 'dart:_js_primitives' as jsPrimitives;
+
 String _symbolToString(Symbol symbol) => _symbol_dev.Symbol.getName(symbol);
 
 _symbolMapToStringMap(Map<Symbol, dynamic> map) {
   if (map == null) return null;
   var result = new Map<String, dynamic>();
   map.forEach((Symbol key, value) {
     result[_symbolToString(key)] = value;
   });
   return result;
 }
@@ skipping 298 matching lines @@
 
 patch class Uri {
   patch static bool get _isWindows => false;
 
   patch static Uri get base {
     String uri = Primitives.currentUri();
     if (uri != null) return Uri.parse(uri);
     throw new UnsupportedError("'Uri.base' is not supported");
   }
 }
+
+patch void get breakpoint {
+  jsPrimitives.breakpoint();
+}