Add 'dart:debugger' library

runtime/lib/debugger.dart

Index: runtime/lib/debugger.dart
diff --git a/runtime/observatory/lib/src/debugger/debugger.dart b/runtime/lib/debugger.dart
similarity index 53%
copy from runtime/observatory/lib/src/debugger/debugger.dart
copy to runtime/lib/debugger.dart
index b2d5203aff7384e1c5b2e269c38e815a060b1145..8e381cad1602a32f67e2fb1d4738dcb8489423b6 100644
--- a/runtime/observatory/lib/src/debugger/debugger.dart
+++ b/runtime/lib/debugger.dart
@@ -2,12 +2,9 @@
 // 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.
 
-part of debugger;
+import 'dart:_internal';
 
-// TODO(turnidge): Move more of ObservatoryDebugger to this class.
-abstract class Debugger {
-  VM get vm;
-  Isolate get isolate;
-  ServiceMap get stack;
-  int get currentFrame;
+patch class Debugger {
+  /* patch */ static void breakHere() native "Debugger_breakHere";
+  /* patch */ static void breakHereIf(bool expr) native "Debugger_breakHereIf";
 }