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

dart/sdk/lib/_internal/lib/core_patch.dart

Index: dart/sdk/lib/_internal/lib/core_patch.dart
diff --git a/dart/sdk/lib/_internal/lib/core_patch.dart b/dart/sdk/lib/_internal/lib/core_patch.dart
index 30aa9264d8b96b8f071145c78eae3f3040973a62..be89b2f9cfc01ff059f094560bc86e0a1e719887 100644
--- a/dart/sdk/lib/_internal/lib/core_patch.dart
+++ b/dart/sdk/lib/_internal/lib/core_patch.dart
@@ -12,6 +12,8 @@ import 'dart:_js_helper' show checkNull,
                               stringJoinUnchecked,
                               objectHashCode;
 
+import 'dart:_js_primitives' as jsPrimitives;
+
 String _symbolToString(Symbol symbol) => _symbol_dev.Symbol.getName(symbol);
 
 _symbolMapToStringMap(Map<Symbol, dynamic> map) {
@@ -330,3 +332,7 @@ patch class Uri {
     throw new UnsupportedError("'Uri.base' is not supported");
   }
 }
+
+patch void get breakpoint {
+  jsPrimitives.breakpoint();
+}