Add warning messages to test.dart to help debug vm tests on Windows.

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 b5febc10522f67ef4b0cebc9f8f965894b938076..9599fa370e3f22e41fa7b65a35eabdf63ab348bc 100644
--- a/tools/testing/dart/multitest.dart
+++ b/tools/testing/dart/multitest.dart
@@ -159,9 +159,11 @@ String CreateMultitestDirectory(String outputDir, String testDir) {
   final String generatedTestDirectory = 'generated_tests';
   Directory parentDir = new Directory(outputDir + generatedTestDirectory);
   if (!new Directory(outputDir).existsSync()) {
+    print('test.dart: build dir does not exist: $outputDir');
     new Directory(outputDir).createSync();
   }
   if (!parentDir.existsSync()) {
+    print('test.dart: generated tests dir does not exist: ${parentDir.path}');
     parentDir.createSync();
   }
   var split = testDir.split('/');