Revert "Use native async/await support in pub."

sdk/lib/_internal/pub_generated/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.
 
 library pub.barback.transformer_id;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:source_span/source_span.dart';
@@ skipping 44 matching lines @@
       return new TransformerId(parts.single, null, span);
     }
 
     return new TransformerId(parts.first, parts.last, span);
   }
 
   TransformerId(this.package, this.path, this.span) {
     if (!package.startsWith('\$')) return;
     if (_BUILT_IN_TRANSFORMERS.contains(package)) return;
     throw new SourceSpanFormatException(
-        'Unsupported built-in transformer $package.', span);
+        'Unsupported built-in transformer $package.',
+        span);
   }
 
-  bool operator==(other) =>
+  bool operator ==(other) =>
       other is TransformerId && other.package == package && other.path == path;
 
   int get hashCode => package.hashCode ^ path.hashCode;
 
   /// Returns a serialized form of [this] that can be passed to
   /// [new TransformerId.parse].
   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) {
     if (path != null) {
       return new Future.value(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/$package.dart'),
-            test: (e) => e is AssetNotFoundException);
+    return barback.getAssetById(
+        transformerAsset).then(
+            (_) =>
+                transformerAsset).catchError(
+                    (e) => new AssetId(package, 'lib/$package.dart'),
+                    test: (e) => e is AssetNotFoundException);
   }
 }