Fix code for strong zones.

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:math' as math;
 
 import 'frame.dart';
 import 'lazy_chain.dart';
 import 'stack_zone_specification.dart';
@@ skipping 75 matching lines @@
 
       return runZoned(callback, onError: newOnError);
     }
 
     var spec = new StackZoneSpecification(onError);
     return runZoned(() {
       try {
         return callback();
       } catch (error, stackTrace) {
         // TODO(nweiz): Don't special-case this when issue 19566 is fixed.
-        return Zone.current.handleUncaughtError(error, stackTrace);
+        Zone.current.handleUncaughtError(error, stackTrace);
+        return null;
       }
     },
         zoneSpecification: spec.toSpec(),
         zoneValues: {_specKey: spec, StackZoneSpecification.disableKey: false});
   }
 
   /// If [when] is `true` and this is called within a [Chain.capture] zone, runs
   /// [callback] in a [Zone] in which chain capturing is disabled.
   ///
   /// If [callback] returns a value, it will be returned by [disable] as well.
@@ skipping 133 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 '${frame.location.padRight(longest)}  ${frame.member}\n';
       }).join();
     }).join(chainGap);
   }
 }