Properly detect name collisions at library level

tests/language/src/ToplevelCollision1Test.dart

Index: tests/language/src/ToplevelCollision1Test.dart
===================================================================
--- tests/language/src/ToplevelCollision1Test.dart	(revision 0)
+++ tests/language/src/ToplevelCollision1Test.dart	(revision 0)
@@ -0,0 +1,15 @@
+// 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.
+
+int x = 100;
+
+get x() => 200;                 /// 00: compile-time error
+set x(var i) { print(i); }      /// 01: compile-time error
+
+int x(a, b) { print(a + b); }   /// 02: compile-time error
+
+
+void main() {
+  // No need to reference x.
+}