Fix the prints matcher test on dart2js.

pkg/matcher/test/prints_matcher_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 matcher.prints_matchers_test;
 
 import 'dart:async';
 
 import 'package:matcher/matcher.dart';
 import 'package:unittest/unittest.dart';
 
 import 'test_utils.dart';
 
+/// The VM and dart2js have different toStrings for closures.
+final closureToString = (() {}).toString();
+
 void main() {
   initUtils();
 
   group('synchronous', () {
     test("passes with an expected print", () {
       shouldPass(() => print("Hello, world!"), prints("Hello, world!\n"));
     });
 
     test("combines multiple prints", () {
       shouldPass(() {
         print("Hello");
         print("World!");
       }, prints("Hello\nWorld!\n"));
     });
 
     test("works with a Matcher", () {
       shouldPass(() => print("Hello, world!"), prints(contains("Hello")));
     });
 
     test("describes a failure nicely", () {
       shouldFail(() => print("Hello, world!"), prints("Goodbye, world!\n"),
           "Expected: prints 'Goodbye, world!\\n' ''"
-          "  Actual: <Closure: () => dynamic> "
+          "  Actual: <$closureToString> "
           "   Which: printed 'Hello, world!\\n' ''"
           "   Which: is different. "
           "Expected: Goodbye, w ... "
           "  Actual: Hello, wor ... "
           "          ^ Differ at offset 0");
     });
 
     test("describes a failure with a non-descriptive Matcher nicely", () {
       shouldFail(() => print("Hello, world!"), prints(contains("Goodbye")),
           "Expected: prints contains 'Goodbye'"
-          "  Actual: <Closure: () => dynamic> "
+          "  Actual: <$closureToString> "
           "   Which: printed 'Hello, world!\\n' ''");
     });
 
     test("describes a failure with no text nicely", () {
       shouldFail(() {}, prints(contains("Goodbye")),
           "Expected: prints contains 'Goodbye'"
-          "  Actual: <Closure: () => dynamic> "
+          "  Actual: <$closureToString> "
           "   Which: printed nothing.");
     });
   });
 
   group('asynchronous', () {
     test("passes with an expected print", () {
       shouldPass(() => new Future(() => print("Hello, world!")),
           prints("Hello, world!\n"));
     });
 
@@ skipping 30 matching lines @@
     });
 
     test("describes a failure with no text nicely", () {
       shouldFail(() => new Future.value(), prints(contains("Goodbye")),
           "Expected: contains 'Goodbye'"
           "  Actual: ''",
           isAsync: true);
     });
   });
 }