Invoke dart2js lazily when using pub serve.

sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart

Index: sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
index f6f64c5ec3d9dbe20eecd4b840a5e73430da57ed..84ca33618cb4c4b08c8ed2085ecb4d744ab9eeb9 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
@@ -20,8 +20,6 @@ import '../../../../compiler/implementation/source_file.dart';
 import '../barback.dart';
 import '../dart.dart' as dart;
 import '../io.dart';
-import '../package.dart';
-import '../package_graph.dart';
 import '../utils.dart';
 import 'build_environment.dart';
 
@@ -33,7 +31,7 @@ final _validOptions = new Set<String>.from([
 
 /// A [Transformer] that uses dart2js's library API to transform Dart
 /// entrypoints in "web" to JavaScript.
-class Dart2JSTransformer extends Transformer {
+class Dart2JSTransformer extends Transformer implements LazyTransformer {
   final BuildEnvironment _environment;
   final BarbackSettings _settings;
 
@@ -139,6 +137,14 @@ class Dart2JSTransformer extends Transformer {
     });
   }
 
+  Future declareOutputs(DeclaringTransform transform) {
+    var primaryId = transform.primaryInput.id;
+    transform.declareOutput(primaryId.addExtension(".js"));
+    transform.declareOutput(primaryId.addExtension(".js.map"));
+    transform.declareOutput(primaryId.addExtension(".precompiled.js"));
+    return new Future.value();
+  }
+
   /// Parses and returns the "commandLineOptions" configuration option.
   List<String> get _configCommandLineOptions {
     if (!_settings.configuration.containsKey('commandLineOptions')) return null;