Add 'dart:debugger' library

sdk/lib/debugger/debugger.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.
+
+/// Programmatically trigger breakpoints.
+library dart.debugger;
+
+/// Programmatically trigger breakpoints.
+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. The behaviour is the same regardless of whether or
+  /// not a debugger is connected.
+  static void breakHere() {
+    print('Debugger.breakHere not supported on this platform.');
+  }
+
+  /// If [expr] is true, stop the program as if a breakpoint where hit at the
+  /// following statement.
+  static void breakHereIf(bool expr) {
+    print('Debugger.breakHereIf not supported on this platform.');
+  }
+}