Disallow assignment to a top level method in dartc

tests/language/src/AssignTopMethodNegativeTest.dart

Index: tests/language/src/AssignTopMethodNegativeTest.dart
diff --git a/tests/language/src/AssignTopMethodNegativeTest.dart b/tests/language/src/AssignTopMethodNegativeTest.dart
new file mode 100644
index 0000000000000000000000000000000000000000..265b204bcaab32b3604e14ef544224782caf8fb4
--- /dev/null
+++ b/tests/language/src/AssignTopMethodNegativeTest.dart
@@ -0,0 +1,10 @@
+// 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.
+
+method() { return 0; }
+
+main() {
+  // Illegal, can't change a top level method
+  method = () { return 1; };
+}