Improve toString on mirrors and align VM and dart2js.

dart/tests/lib/mirrors/to_string_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.
+
+library test.to_string_test;
+
+import 'dart:mirrors';
+
+import 'package:expect/expect.dart';
+
+expect(expected, actual) => Expect.stringEquals(expected, '$actual');
+
+class Foo {
+  var field;
+  method() {}
+}
+
+main() {
+  var mirrors = currentMirrorSystem();
+  expect("TypeMirror on 'dynamic'", mirrors.dynamicType);
+  expect("TypeMirror on 'void'", mirrors.voidType);
+  expect("LibraryMirror on 'test.to_string_test'",
+         mirrors.findLibrary(const Symbol('test.to_string_test')).single);
+  expect("InstanceMirror on 1", reflect(1));
+  expect("ClassMirror on 'Foo'", reflectClass(Foo));
+  expect("VariableMirror on 'field'",
+         reflectClass(Foo).variables.values.single);
+  expect("MethodMirror on 'method'", reflectClass(Foo).methods.values.single);
+  String s = reflect(main).toString();
+  Expect.isTrue(s.startsWith("ClosureMirror on '"), s);
+}