Infer types for compile-time constant closures.

tests/compiler/dart2js/simple_inferrer_global_field_closure_test.dart

 // Copyright (c) 2014, 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.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'compiler_helper.dart';
 
 const String TEST = """
 
 method1() {
   return 42;
 }
 
 method2(a) {  // Called only via [foo2] with a small integer.
   return a;
 }
 
-const foo1 = method1;
-const foo2 = method2;
+var foo1 = method1;
+var foo2 = method2;
 
 returnInt1() {
   return foo1();
 }
 
 returnInt2() {
   return foo2(54);
 }
 
 main() {
@@ skipping 16 matching lines @@
           name);
     }
 
     checkReturn('method1', compiler.typesTask.uint31Type);
     checkReturn('returnInt1', compiler.typesTask.uint31Type);
 
     checkReturn('method2', compiler.typesTask.uint31Type);
     checkReturn('returnInt2', compiler.typesTask.uint31Type);
   }));
 }