Fix all strong-mode warnings.

lib/src/errors.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.errors;
 
 import 'package:stack_trace/stack_trace.dart';
 
 import 'asset/asset_id.dart';
 import 'transformer/wrapping_aggregate_transformer.dart';
 import 'utils.dart';
 
 /// Error thrown when an asset with [id] cannot be found.
 class AssetNotFoundException implements Exception {
   final AssetId id;
 
   AssetNotFoundException(this.id);
 
   String toString() => "Could not find asset $id.";
 }
 
 /// Replaces any occurrences of [AggregateException] in [errors] with the list
 /// of errors it contains.
 Iterable<BarbackException> flattenAggregateExceptions(
     Iterable<BarbackException> errors) {
   return errors.expand((error) {
-    if (error is! AggregateException) return [error];
-    return error.errors;
+    if (error is AggregateException) return error.errors;
+    return [error];
   });
 }
 
 /// The interface for exceptions from the barback graph or its transformers.
 ///
 /// These exceptions are never produced by programming errors in barback.
 abstract class BarbackException implements Exception {
   /// Takes a collection of [BarbackExceptions] and returns a single exception
   /// that contains them all.
   ///
@@ skipping 144 matching lines @@
 
   bool operator==(other) =>
       other is TransformInfo &&
       other.transformer == transformer &&
       other.primaryId == primaryId;
 
   int get hashCode => transformer.hashCode ^ primaryId.hashCode;
 
   String toString() => "$transformer on $primaryId";
 }