Fix bug in allowInteropCaptureThis that caused JavaScript objects to accidentally be wrapped.

sdk/lib/js/dart2js/js_dart2js.dart

 // Copyright (c) 2013, 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.
 
 /**
  * Support for interoperating with JavaScript.
  *
  * This library provides access to JavaScript objects from Dart, allowing
  * Dart code to get and set properties, and call methods of JavaScript objects
  * and invoke JavaScript functions. The library takes care of converting
@@ skipping 686 matching lines @@
   JS('', '#.# = #', ret, DART_CLOSURE_PROPERTY_NAME, f);
   JS('', '#.# = #', f, _JS_FUNCTION_PROPERTY_NAME_CAPTURE_THIS, ret);
   return ret;
 }
 
 _callDartFunctionFast(callback, List arguments) {
   return Function.apply(callback, arguments);
 }
 
 _callDartFunctionFastCaptureThis(callback, self, List arguments) {
-  return _convertToJS(Function.apply(callback, [self]..addAll(arguments)));
+  return Function.apply(callback, [self]..addAll(arguments));
 }
 
 Function allowInterop(Function f) {
   if (JS('bool', 'typeof(#) == "function"', f)) {
     // Already supports interop, just use the existing function.
     return f;
   } else {
     return _convertDartFunctionFast(f);
   }
 }
 
 Function allowInteropCaptureThis(Function f) {
   if (JS('bool', 'typeof(#) == "function"', f)) {
     // Behavior when the function is already a JS function is unspecified.
     throw new ArgumentError(
         "Function is already a JS function so cannot capture this.");
     return f;
   } else {
     return _convertDartFunctionFastCaptureThis(f);
   }
 }