Refactor handling of unqualified static updates.

tests/compiler/dart2js/resolver_test.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.
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/constants/expressions.dart';
@@ skipping 1209 matching lines @@
       main() {
         mname() { };
         mname = 3;
       }
       ''', [MessageKind.ASSIGNING_METHOD]);
 
   // Can't override top-level functions
   checkWarningOn('''
       m() {}
       main() { m = 4; }
-      ''', [MessageKind.ASSIGNING_METHOD]);
+      ''', [MessageKind.ASSIGNING_METHOD,
+            // TODO(johnniwinther): Avoid duplicate warnings.
+            MessageKind.NOT_ASSIGNABLE]);
 
   // Can't override instance methods
   checkWarningOn('''
       main() { new B().bar(); }
       class B {
         mname() {}
         bar() {
           mname = () => null;
         }
       }
@@ skipping 171 matching lines @@
     }
     main() => A.m();
   ''', functionName: 'm');
   check('''
     class A {
       m() => () => await - 3;
     }
     main() => new A().m();
   ''', className: 'A');
 }