Remove deprecated parts of dart:async.

pkg/unittest/lib/src/future_matchers.dart

Index: pkg/unittest/lib/src/future_matchers.dart
diff --git a/pkg/unittest/lib/src/future_matchers.dart b/pkg/unittest/lib/src/future_matchers.dart
index 490ca6af88411238e627fec1ebc54710de28c416..c9e2742ec5ba737e58dff5f6360dd4238cb5bbcf 100644
--- a/pkg/unittest/lib/src/future_matchers.dart
+++ b/pkg/unittest/lib/src/future_matchers.dart
@@ -42,11 +42,10 @@ class _Completes extends Matcher {
 
     item.then((value) {
       done(() { if (_matcher != null) expect(value, _matcher); });
-    }, onError: (error) {
+    }, onError: (error, trace) {
       var id = _id == '' ? '' : '${_id} ';
       var reason = 'Expected future ${id}to complete successfully, '
                    'but it failed with ${error}';
-      var trace = getAttachedStackTrace(error);
       if (trace != null) {
         var stackTrace = trace.toString();
         stackTrace = '  ${stackTrace.replaceAll('\n', '\n  ')}';