Remove deprecated Strings class.

utils/testrunner/pipeline_utils.dart

Index: utils/testrunner/pipeline_utils.dart
diff --git a/utils/testrunner/pipeline_utils.dart b/utils/testrunner/pipeline_utils.dart
index 4362e3d59fcaff2cd60abe71373706e2150d33d6..94391dca766d582a42e9e042054180cfd10fed71 100644
--- a/utils/testrunner/pipeline_utils.dart
+++ b/utils/testrunner/pipeline_utils.dart
@@ -61,7 +61,7 @@ void writeFile(String fileName, String contents) {
 Future _processHelper(String command, List<String> args,
     [int timeout = 300, int procId = 0, Function outputMonitor]) {
   var completer = procId == 0 ? new Completer() : null;
-  log.add('Running $command ${Strings.join(args, " ")}');
+  log.add('Running $command ${args.join(" ")}');
   var timer = null;
   var stdoutHandler, stderrHandler;
   var processFuture = Process.start(command, args);