Fix more strong mode errors and warnings.

lib/src/barback/transformer_id.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 'package:barback/barback.dart';
 import 'package:source_span/source_span.dart';
 
 import '../utils.dart';
@@ skipping 62 matching lines @@
   String serialize() => path == null ? package : '$package/$path';
 
   String toString() => serialize();
 
   /// Returns the asset id for the library identified by this transformer id.
   ///
   /// If `path` is null, this will determine which library to load. Unlike
   /// [getAssetId], this doesn't take generated assets into account; it's used
   /// to determine transformers' dependencies, which requires looking at files
   /// on disk.
-  Future<AssetId> getAssetId(Barback barback) {
+  Future<AssetId> getAssetId(Barback barback) async {
     if (path != null) {
-      return new Future.value(new AssetId(package, 'lib/$path.dart'));
+      return new AssetId(package, 'lib/$path.dart');
     }
 
     var transformerAsset = new AssetId(package, 'lib/transformer.dart');
-    return barback.getAssetById(transformerAsset).then((_) => transformerAsset)
-        .catchError((e) => new AssetId(package, 'lib/$package.dart'),
-            test: (e) => e is AssetNotFoundException);
+    try {
+      await barback.getAssetById(transformerAsset);
+      return transformerAsset;
+    } on AssetNotFoundException catch (_) {
+      return new AssetId(package, 'lib/$package.dart');
+    }
   }
 }