Correctly handle the --arch argument to dart test scripts.

tools/testing/dart/multitest.dart

Index: tools/testing/dart/multitest.dart
diff --git a/tools/testing/dart/multitest.dart b/tools/testing/dart/multitest.dart
index 2a92eed4ec8dc838f2378979a131a0127913f35d..79866789f43723b7bfb6b1b9362235f2842ca06b 100644
--- a/tools/testing/dart/multitest.dart
+++ b/tools/testing/dart/multitest.dart
@@ -198,14 +198,14 @@ WriteMultitestToFileAndQueueIt(Map<String, String> tests,
 
 String CreateMultitestDirectory(String buildDir, String testDir) {
   final String generatedTestDirectory = 'generated_tests/';
-  Directory parent_dir = new Directory(buildDir + generatedTestDirectory);
-  if (!parent_dir.existsSync()) {
-    parent_dir.createSync();
+  Directory parentDir = new Directory(buildDir + generatedTestDirectory);
+  if (!parentDir.existsSync()) {
+    parentDir.createSync();
   }
   var split = testDir.split(new Platform().pathSeparator());
   var lastComponent = split.removeLast();
   Expect.isTrue(lastComponent == 'src');
-  String path = parent_dir.path + split.last();
+  String path = parentDir.path + split.last();
   Directory dir = new Directory(path);
   if (!dir.existsSync()) {
     dir.createSync();