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

sdk/lib/_internal/pub_generated/lib/src/barback/transformer_id.dart

Index: sdk/lib/_internal/pub_generated/lib/src/barback/transformer_id.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/transformer_id.dart b/sdk/lib/_internal/pub_generated/lib/src/barback/transformer_id.dart
similarity index 90%
copy from sdk/lib/_internal/pub/lib/src/barback/transformer_id.dart
copy to sdk/lib/_internal/pub_generated/lib/src/barback/transformer_id.dart
index c36d4ae24cedb22d74fd74c6f172b93abf6db151..2017d9bb0a8cd6305545b51cc37836e8dfb880a9 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/transformer_id.dart
+++ b/sdk/lib/_internal/pub_generated/lib/src/barback/transformer_id.dart
@@ -62,10 +62,11 @@ class TransformerId {
     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;
@@ -88,8 +89,11 @@ class TransformerId {
     }
 
     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);
   }
 }