Fix a regression in ObjectMirror.setFieldAsync.

runtime/lib/mirrors_impl.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.
 
 // VM-specific implementation of the dart:mirrors library.
 
 // These values are allowed to be passed directly over the wire.
 bool _isSimpleValue(var value) {
   return (value == null || value is num || value is String || value is bool);
 }
@@ skipping 208 matching lines @@
     } on MirroredError catch(e) {
       return new Future.error(e);
     }
   }
 
   Future<InstanceMirror> setFieldAsync(Symbol memberName, Object value) {
     try {
       var unwrappedValue;
       if(_isSimpleValue(value)) {
         unwrappedValue = value;
-      } else if(wrappedArg is InstanceMirror) {
+      } else if(value is InstanceMirror) {
         unwrappedValue = value._reflectee;
       } else { 
-        throw "setter argument ($value) must be a simple value or InstanceMirror";
+        throw "setter argument ($value) must be"
+              "a simple value or InstanceMirror";
       }
 
       var result = this._invokeSetter(_reflectee,
                                       _n(memberName),
                                       unwrappedValue);
       return new Future.value(reflect(result)); 
     } on MirroredError catch(e) {
       return new Future.error(e);
     }
   }
@@ skipping 919 matching lines @@
   static Expando<ClassMirror> _classMirrorCache = new Expando("ClassMirror");
   static ClassMirror reflectClass(Type key) {
     var classMirror = _classMirrorCache[key];
     if (classMirror == null) {
       classMirror = makeLocalClassMirror(key);
       _classMirrorCache[key] = classMirror;
     }
     return classMirror;
   }
 }