dart2js cps: Fix return type for native functions.

pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.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.
 
 library dart2js.ir_builder_task;
 
 import '../closure.dart' as closure;
 import '../common.dart';
 import '../common/names.dart' show
     Names,
@@ skipping 3526 matching lines @@
 
   DartType unaliasType(DartType type) => type.unaliased;
 
   TypeMask getTypeMaskForForeign(NativeBehavior behavior) {
     if (behavior == null) {
       return _backend.dynamicType;
     }
     return TypeMaskFactory.fromNativeBehavior(behavior, _compiler);
   }
 
+  TypeMask getTypeMaskForNativeFunction(FunctionElement function) {
+    return _compiler.typesTask.getGuaranteedTypeOfElement(function);
+  }
+
   FieldElement locateSingleField(Selector selector, TypeMask type) {
     return _compiler.world.locateSingleField(selector, type);
   }
 
   Element get closureConverter {
     return _backend.helpers.closureConverter;
   }
 
   void addNativeMethod(FunctionElement function) {
     _backend.emitter.nativeEmitter.nativeMethods.add(function);
   }
 }