Provide a dart2js implementation of dart:debugger

sdk/lib/_internal/compiler/js_lib/debugger_patch.dart

+// Copyright (c) 2015, 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:debugger library.
+
+import 'dart:_js_helper' show patch;
+import 'dart:_foreign_helper' show JS;
+
+@patch class Debugger {
+  /// Stop the program as if a breakpoint where hit at the following statement.
+  /// NOTE: When invoked, the isolate will not return until a debugger
+  /// continues execution. When running in the Dart VM the behaviour is the same
+  /// regardless of whether or not a debugger is connected. When compiled to
+  /// JavaScript, this uses the "debugger" statement, and behaves exactly as
+  /// that does.
+  @patch
+  @ForceInline()
+  static void breakHere() {
+    JS('', 'debugger');
+  }
+
+  /// If [expr] is true, stop the program as if a breakpoint where hit at the
+  /// following statement. See [breakHere].
+  @patch
+  @ForceInline()
+  static void breakHereIf(bool expr) {
+    if (expr) {
+      breakHere();  
+    }
+  }
+}