Fix all strong mode warnings.

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 175 matching lines @@
       return new Chain([foldedTraces.last]);
     }
 
     return new Chain(nonEmptyTraces);
   }
 
   /// Converts [this] to a [Trace].
   ///
   /// The trace version of a chain is just the concatenation of all the traces
   /// in the chain.
-  Trace toTrace() => new Trace(flatten(traces.map((trace) => trace.frames)));
+  Trace toTrace() => new Trace(traces.expand((trace) => trace.frames));
 
   String toString() {
     // Figure out the longest path so we know how much to pad.
     var longest = traces.map((trace) {
       return trace.frames.map((frame) => frame.location.length)
           .fold(0, math.max);
     }).fold(0, math.max);
 
     // 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);
   }
 }