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

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

Index: pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
index 84b8e31e0bac0c4e02a1daf3e693e9bf5b3b81bf..4649295cbae6df1114cfb50e9167700cef99305b 100644
--- a/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
@@ -420,6 +420,11 @@ class Emitter implements js_emitter.Emitter {
     }
   }
 
+  @override
+  int generatedSize(OutputUnit unit) {
+    return outputBuffers[unit].length;
+  }
+
   List<jsAst.Statement> buildTrivialNsmHandlers() {
     return nsmEmitter.buildTrivialNsmHandlers();
   }