Disallow assignment to a top level method in dartc

tests/language/src/AssignInstanceMethodNegativeTest.dart

 // Copyright (c) 2011, 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.
 
 class A {
   A() {}
   imethod() { return 0; }
 }
 
-class AssignInstanceMethodNegativeTest {
-
-  static testMain() {
-    var a = new A();
-    // Illegal, can't change a member method
-    a.imethod = () { return 1; };
-  }
+main() {
+  var a = new A();
+  // Illegal, can't change a member method
+  a.imethod = () { return 1; };
 }
-
-main() {
-  AssignInstanceMethodNegativeTest.testMain();
-}