Support !=.

dart/frog/tests/leg_only/src/NotEqualsTest.dart

Index: dart/frog/tests/leg_only/src/NotEqualsTest.dart
===================================================================
--- dart/frog/tests/leg_only/src/NotEqualsTest.dart	(revision 0)
+++ dart/frog/tests/leg_only/src/NotEqualsTest.dart	(revision 0)
@@ -0,0 +1,21 @@
+// 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.
+
+void main() {
+  var x = 3;
+  if (x != x) {
+    throw "x != x with x == 3";
+  } else {
+    print('good');
+  }
+  var y = x;
+  if (x != y) throw "3 != 3";
+
+  var z = 4;
+  if (x != z) {
+    print('good');
+  } else {
+    throw 'x == z';
+  }
+}