Fix broken console evaluation in devtools when stopped at an async frame.

tools/dom/src/native_DOMImplementation.dart

 // Copyright (c) 2012, 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.
 
 part of html;
 
 class _Property {
   _Property(this.name) :
       _hasValue = false,
       writable = false,
@@ skipping 248 matching lines @@
     var sb = new StringBuffer("(");
     addArg(arg, value) {
       arg = stripMemberName(arg);
       if (args.containsKey(arg)) return;
       // We ignore arguments with the name 'this' rather than throwing an
       // exception because Dart_GetLocalVariables includes 'this' and it
       // is more convenient to filter it out here than from C++ code.
       // 'this' needs to be handled by calling Dart_EvaluateExpr with
       // 'this' as the target rather than by passing it as an argument.
       if (arg == 'this') return;
+      // Avoid being broken by bogus ':async_op' local passed in when within
+      // an async method.
+      if (arg.startsWith(':')) return;
       if (args.isNotEmpty) {
         sb.write(", ");
       }
       sb.write("final $arg");
       args[arg] = value;
     }
 
     if (includeCommandLineAPI) {
       addArg("\$consoleVariables", _consoleTempVariables);
 
@@ skipping 830 matching lines @@
   _scheduleImmediateHelper._schedule(callback);
 };
 
 get _pureIsolateScheduleImmediateClosure => ((void callback()) =>
   throw new UnimplementedError("scheduleMicrotask in background isolates "
                                "are not supported in the browser"));
 
 // Class for unsupported native browser 'DOM' objects.
 class _UnsupportedBrowserObject extends DartHtmlDomObject {
 }