Fix build break with dart2js not liking conditional expressions in initializers of const classes.

pkg/unittest/lib/src/future_matchers.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.
 
 part of matcher;
 
 /**
  * Matches a [Future] that completes successfully with a value. Note that this
  * creates an asynchronous expectation. The call to `expect()` that includes
  * this will return immediately and execution will continue. Later, when the
@@ skipping 16 matching lines @@
  * [id] is an optional tag that can be used to identify the completion matcher
  * in error messages.
  */
 Matcher completion(matcher, [String id = '']) =>
     new _Completes(wrapMatcher(matcher), id);
 
 class _Completes extends BaseMatcher {
   final Matcher _matcher;
   final String _id;
 
-  const _Completes(this._matcher, String id)
-      : this._id = (id == '') ? '' : '$id ';
+  const _Completes(this._matcher, this._id);
 
   bool matches(item, MatchState matchState) {
     if (item is! Future) return false;
     var done = wrapAsync((fn) => fn(), _id);
 
     item.then((value) {
       done(() { if (_matcher != null) expect(value, _matcher); });
     }, onError: (e) {
-      var reason = 'Expected future ${_id}to complete successfully, '
+      var id = _id == '' ? '' : '${_id} ';

[Siggi Cherem (dart-lang), 2013/03/01 22:17:15]

I was actually going to suggest this anyways, but just for taste reasons =)

+      var reason = 'Expected future ${id}to complete successfully, '
                    'but it failed with ${e.error}';
       if (e.stackTrace != null) {
         var stackTrace = e.stackTrace.toString();
         stackTrace = '  ${stackTrace.replaceAll('\n', '\n  ')}';
         reason = '$reason\nStack trace:\n$stackTrace';
       }
       done(() => fail(reason));
     });
 
     return true;
   }
 
   Description describe(Description description) {
     if (_matcher == null) {
       description.add('completes successfully');
     } else {
       description.add('completes to a value that ').addDescriptionOf(_matcher);
     }
     return description;
   }
 }