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

pkg/unittest/lib/src/future_matchers.dart

Index: pkg/unittest/lib/src/future_matchers.dart
===================================================================
--- pkg/unittest/lib/src/future_matchers.dart	(revision 19339)
+++ pkg/unittest/lib/src/future_matchers.dart	(working copy)
@@ -34,8 +34,7 @@
   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;
@@ -44,7 +43,8 @@
     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();