enable --dump-info with the --fast-startup emitter

pkg/compiler/lib/src/js_emitter/startup_emitter/emitter.dart

Index: pkg/compiler/lib/src/js_emitter/startup_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/startup_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/startup_emitter/emitter.dart
index 617fea3170e63a20f05fde19886b524ac7571f41..d71963f75fbaf97472c62e1e764de6b99520d220 100644
--- a/pkg/compiler/lib/src/js_emitter/startup_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/startup_emitter/emitter.dart
@@ -10,6 +10,7 @@ import 'package:js_runtime/shared/embedded_names.dart'
 import '../../common.dart';
 import '../../compiler.dart' show Compiler;
 import '../../constants/values.dart' show ConstantValue;
+import '../../deferred_load.dart' show OutputUnit;
 import '../../elements/elements.dart'
     show ClassElement, Element, FieldElement, FunctionElement;
 import '../../js/js.dart' as js;
@@ -195,5 +196,12 @@ class Emitter implements emitterTask.Emitter {
   }
 
   @override
+  int generatedSize(OutputUnit unit) {
+    Fragment key = _emitter.outputBuffers.keys
+        .firstWhere((Fragment fragment) => fragment.outputUnit == unit);
+    return _emitter.outputBuffers[key].length;
+  }
+
+  @override
   void invalidateCaches() {}
 }