Implement Evaluate without creating new classes.

tests/lib/mirrors/eval_source_test.dart

 // Copyright (c) 2014, 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.
 
-library test.runtime_type_test;
-
-@MirrorsUsed(targets: 'test.runtime_type_test', override: '*')
 import 'dart:mirrors';
-
 import 'package:expect/expect.dart';
 
-class A {}
-
-class B {
-  get runtimeType => A;
-}
-
 main() {
-  Expect.equals(reflect(new B()).type, reflectClass(B));
-}
+  String expression = "() => 3 + 4";
+  var f = reflect(0).eval(expression);
+  Expect.isTrue(f is Function);
+  Expect.equals(7, f());
+  Expect.stringEquals(expression, reflect(f).function.source);
+}