Provide a dart2js implementation of dart:debugger

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.

[Cutch, 2015/05/06 19:50:10]

DBC: Update comment here.

[Alan Knight, 2015/05/06 22:30:50]

Already done from an earlier comment.

[skybrian, 2015/05/06 22:51:56]

There are doc comments in two places, here and in debugger_patch.dart. Which one
shows up on api.dartlang.org and in IDE's? If it's this version, then perhaps
this comment needs to be updated as well?

-  static void breakHere() {
-    print('Debugger.breakHere not supported on this platform.');
-  }
+  external static void breakHere();
 
   /// 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.');
-  }
+  external static void breakHereIf(bool expr);
 }