Add mirrors tests

test/codegen/lib/mirrors/disable_tree_shaking_test.dart

+// Copyright (c) 2013, 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.
+
+// Ensure that reflection works on methods that would otherwise be
+// tree-shaken away.
+
+import "dart:mirrors";
+
+class Foo {
+  Foo();
+  foo() => 42;
+}
+
+main() {
+  // Do NOT instantiate Foo.
+  var m = reflectClass(Foo);
+  var instanceMirror = m.newInstance(new Symbol(''), []);
+  var result = instanceMirror.invoke(new Symbol('foo'), []).reflectee;
+  if (result != 42) {
+    throw 'Expected 42, but got $result';
+  }
+}