Fix warnings.

sdk/lib/_internal/pub/lib/src/barback.dart

Index: sdk/lib/_internal/pub/lib/src/barback.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback.dart b/sdk/lib/_internal/pub/lib/src/barback.dart
index 2d0b6006180c62ac035aa6c2760f9983ee1becf2..803ec64fb69a049146313b9d4a05aaa3017c63fe 100644
--- a/sdk/lib/_internal/pub/lib/src/barback.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback.dart
@@ -10,9 +10,9 @@ import 'package:barback/barback.dart';
 
 import 'barback/load_all_transformers.dart';
 import 'barback/pub_package_provider.dart';
-import 'barback/rewrite_import_transformer.dart';
 import 'barback/server.dart';
 import 'barback/watch_sources.dart';
+import 'package_graph.dart';
 import 'utils.dart';
 
 /// An identifier for a transformer and the configuration that will be passed to
@@ -97,7 +97,7 @@ Future<BarbackServer> createServer(String host, int port, PackageGraph graph) {
 /// Otherwise, it expands to lib/${path}.dart in that package.
 AssetId libraryIdentifierToId(String identifier) {
   if (identifier.isEmpty) {
-    throw new FormatError('Invalid library identifier: "".');
+    throw new FormatException('Invalid library identifier: "".');
   }
 
   // Convert the concise asset name in the pubspec (of the form "package"