Change output of string equality match failures.

pkg/unittest/test/matchers_test.dart

 // Copyright (c) 2012, 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.
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:unittest/unittest.dart';
 
 import 'test_common.dart';
@@ skipping 488 matching lines @@
       shouldFail(a, containsValue('ba'),
           "Expected: contains value 'ba' But: was {'foo': 'bar'}.");
     });
 
     test('containsPair', () {
       var a = new Map();
       a['foo'] = 'bar';
       shouldPass(a, containsPair('foo', 'bar'));
       shouldFail(a, containsPair('foo', 'ba'),
           "Expected: contains pair 'foo' => 'ba' "
-          "But:  contains key 'foo' but with value was 'bar'. "
+          "But: Strings are not equal. Both strings start the same, "
+          "but the given value also has the following trailing characters: r. "
           "Actual: {'foo': 'bar'}");
       shouldFail(a, containsPair('fo', 'bar'),
           "Expected: contains pair 'fo' => 'bar' "
           "But: doesn't contain key 'fo'. "
           "Actual: {'foo': 'bar'}");
     });
 
     test('hasLength', () {
       var a = new Map();
       a['foo'] = 'bar';
@@ skipping 71 matching lines @@
     test('throwsA - correct error', () {
       var completer = new Completer();
       completer.completeError('X');
       shouldPass(completer.future, throwsA(equals('X')), isAsync: true);
     });
 
     test('throwsA - wrong error', () {
       var completer = new Completer();
       completer.completeError('X');
       shouldFail(completer.future, throwsA(equals('Y')),
-          "Expected: 'Y' But: was 'X'.",
+          "Expected: 'Y' But: Strings are not equal. "
+          "Expected: Y But was: XX ^ Differ at position 0. Actual: 'X'",

[Siggi Cherem (dart-lang), 2013/05/22 23:09:42]

I assume some \n and spaces are trimmed here? it would be good to test that the
output is actually aligned correctly

           isAsync: true);
     });
   });
 
   group('Predicate Matchers', () {
     test('isInstanceOf', () {
       shouldFail(0, predicate((x) => x is String, "an instance of String"),
           "Expected: an instance of String But: was <0>.");
       shouldPass('cow', predicate((x) => x is String, "an instance of String"));
     });
   });
 }