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.
 
-/// Programmatically trigger breakpoints.
-library dart.debugger;
+// Patch file for dart:debugger library.
 
-/// Programmatically trigger breakpoints.
-class Debugger {
+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. The behaviour is the same regardless of whether or
   /// not a debugger is connected.

[skybrian, 2015/05/05 19:35:38]

The last statement isn't true for the JavaScript "debugger" statement [1], so I
think we need to qualify it? "When running in the Dart VM, the behavior is the
same [...]".

[1]
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/...

[sra1, 2015/05/06 04:33:10]

+1

[Alan Knight, 2015/05/06 17:04:39]

Done.

+  @patch
   static void breakHere() {

[floitsch, 2015/05/05 20:29:28]

The _js_helper library has a "ForceInline()" annotation.
I would put it on this method.

[Alan Knight, 2015/05/06 17:04:39]

Done.

-    print('Debugger.breakHere not supported on this platform.');
+    JS('', 'debugger');
   }
 
   /// If [expr] is true, stop the program as if a breakpoint where hit at the
   /// following statement.
+  @patch
   static void breakHereIf(bool expr) {

[floitsch, 2015/05/05 20:29:28]

ditto.

[Alan Knight, 2015/05/06 17:04:38]

Done.

-    print('Debugger.breakHereIf not supported on this platform.');
+    if (expr) {
+      breakHere();  
+    }
   }
-}
+}