Fix a syntax error.

lib/src/chain.dart

 // Copyright (c) 2013, 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 'dart:math' as math;
 
 import 'frame.dart';
 import 'stack_zone_specification.dart';
@@ skipping 50 matching lines @@
   /// error. Note that if [callback] produces multiple unhandled errors,
   /// [onError] may be called more than once. If [onError] isn't passed, the
   /// parent Zone's `unhandledErrorHandler` will be called with the error and
   /// its chain.
   ///
   /// Note that even if [onError] isn't passed, this zone will still be an error
   /// zone. This means that any errors that would cross the zone boundary are
   /// considered unhandled.
   ///
   /// If [callback] returns a value, it will be returned by [capture] as well.
-  static /*=T*? capture/*<T>*/(/*=T*/ callback(),
+  static /*=T*/ capture/*<T>*/(/*=T*/ callback(),
       {void onError(error, Chain chain), bool when: true}) {
     if (!when) {
       var newOnError;
       if (onError != null) {
         newOnError = (error, stackTrace) {
           onError(error, new Chain.forTrace(stackTrace));
         };
       }
 
       return runZoned(callback, onError: newOnError);
@@ skipping 122 matching lines @@
 
     // Don't call out to [Trace.toString] here because that doesn't ensure that
     // padding is consistent across all traces.
     return traces.map((trace) {
       return trace.frames.map((frame) {
         return '${padRight(frame.location, longest)}  ${frame.member}\n';
       }).join();
     }).join(chainGap);
   }
 }