Add 'dart:debugger' library

runtime/lib/debugger.cc

 // Copyright (c) 2014, 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.
 
 #include "vm/bootstrap_natives.h"
 
+#include "include/dart_api.h"
+
+#include "vm/debugger.h"
+#include "vm/exceptions.h"
 #include "vm/native_entry.h"
 #include "vm/object.h"
+#include "vm/object_store.h"
 
 namespace dart {
 
-DEFINE_NATIVE_ENTRY(Num_toString, 1) {
-  const Number& number = Number::CheckedHandle(arguments->NativeArgAt(0));
-  Heap::Space space = isolate->heap()->ShouldPretenure(kOneByteStringCid) ?
-      Heap::kPretenured : Heap::kNew;
-  return number.ToString(space);
+// dart:debugger.
+
+DEFINE_NATIVE_ENTRY(Debugger_breakpoint, 0) {
+  Debugger* debugger = isolate->debugger();
+  if (!debugger) {
+    return Object::null();
+  }
+  debugger->ExplicitBreakpoint();
+  return Object::null();
 }
 
+
 }  // namespace dart