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

pkg/compiler/lib/src/js_emitter/code_emitter_task.dart

Index: pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
diff --git a/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart b/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
index 0cc5b97f16e54d0f90c015ec997280d9a1b4328d..8ba85a23f8e013f8b113499f8d1029f6d9c01805 100644
--- a/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
+++ b/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
@@ -228,5 +228,8 @@ abstract class Emitter {
   /// Returns the JS template for the given [builtin].
   jsAst.Template templateForBuiltin(JsBuiltin builtin);
 
+  /// Returns the size of the code generated for a given output [unit].
+  int generatedSize(OutputUnit unit);
+
   void invalidateCaches();
 }