dart2js: Add a header to the output of the 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 60382c168c88685200eede50b1df1a4b7a497afb..9c615ff451ad5b98312b25fd640c54a73dd99f01 100644
--- a/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
@@ -12,6 +12,8 @@ import 'package:js_runtime/shared/embedded_names.dart' show
     JsBuiltin,
     JsGetName;
 
+import '../headers.dart';
+
 import '../js_emitter.dart' hide Emitter;
 import '../js_emitter.dart' as js_emitter show Emitter;
 
@@ -2072,11 +2074,7 @@ function(originalDescriptor, name, holder, isStatic, globalFunctionsAccess) {
   }
 
   jsAst.Comment buildGeneratedBy() {
-    String suffix = '';
-    if (compiler.hasBuildId) suffix = ' version: ${compiler.buildId}';
-    String msg = '// Generated by dart2js, the Dart to JavaScript '
-                 'compiler$suffix.';
-    return new jsAst.Comment(msg);
+    return new jsAst.Comment(generatedBy(compiler));
   }
 
   void outputSourceMap(CodeOutput output,