Fix all strong-mode warnings.

lib/src/build_result.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.
 
 library barback.build_result;
 
 import 'errors.dart';
 import 'utils.dart';
 
 /// An event indicating that the cascade has finished building all assets.
@@ skipping 26 matching lines @@
   factory BuildResult.aggregate(Iterable<BuildResult> results) {
     var errors = unionAll(results.map((result) => result.errors));
     return new BuildResult(errors);
   }
 
   String toString() {
     if (succeeded) return "success";
 
     return "errors:\n" + errors.map((error) {
       var stackTrace = null;
-      if (error is TransformerException || error is AssetLoadException) {
+      if (error is TransformerException) {
+        stackTrace = error.stackTrace.terse;
+      } else if (error is AssetLoadException) {
         stackTrace = error.stackTrace.terse;
       }
 
       var msg = new StringBuffer();
       msg.write(prefixLines(error.toString()));
       if (stackTrace != null) {
         msg.write("\n\n");
         msg.write("Stack chain:\n");
         msg.write(prefixLines(stackTrace.toString()));
       }
       return msg.toString();
     }).join("\n\n");
   }
 }