Pass in "mode" to transformer plugins.

sdk/lib/_internal/pub/test/serve/utils.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library pub_tests;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
@@ skipping 169 matching lines @@
 }
 
 /// Reads lines from pub serve's stdout until it prints the build success
 /// message.
 ///
 /// The schedule will not proceed until the output is found. If not found, it
 /// will eventually time out.
 void waitForBuildSuccess() {
   nextLine() {
     return _pubServer.nextLine().then((line) {
-      if (line.contains("successfully")) return;
+      if (line.contains("successfully")) return null;
 
       // This line wasn't it, so ignore it and keep trying.
       return nextLine();
     });
   }
 
   schedule(nextLine);
 }
 
 /// Schedules opening a web socket connection to the currently running pub
@@ skipping 21 matching lines @@
 /// socket. It omitted, request is JSON encoded to a string first.
 void webSocketShouldReply(request, expectation, {bool encodeRequest: true}) {
   schedule(() => _ensureWebSocket().then((_) {
     if (encodeRequest) request = JSON.encode(request);
     _webSocket.add(request);
     return _webSocketBroadcastStream.first.then((value) {
       expect(JSON.decode(value), expectation);
     });
   }), "send $request to web socket and expect reply that $expectation");
 }