Prevent mutation of final variables

tests/language/final_variable_assignment_test.dart

Index: tests/language/final_variable_assignment_test.dart
===================================================================
--- tests/language/final_variable_assignment_test.dart	(revision 0)
+++ tests/language/final_variable_assignment_test.dart	(revision 0)
@@ -0,0 +1,13 @@
+// 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.
+
+// Test to make sure we catch assignments to final local variables.
+
+main() {
+  final x = 30;
+  x = 0;    /// 01: compile-time error
+  x += 1;   /// 02: compile-time error
+  ++x;      /// 03: compile-time error
+  x++;      /// 04: compile-time error
+}