Make Dart test scripts work for the VM on Windows.

tools/testing/dart/test_options.dart

Index: tools/testing/dart/test_options.dart
diff --git a/tools/testing/dart/test_options.dart b/tools/testing/dart/test_options.dart
index cc23ad428c5249a781040263d6b0668d6e8b50c6..574f49061a29d9d9535533bcf5d0bee866e07307 100644
--- a/tools/testing/dart/test_options.dart
+++ b/tools/testing/dart/test_options.dart
@@ -198,6 +198,14 @@ class TestOptionsParser {
    * into a list of configurations with exactly one value per key.
    */
   List<Map> _expandConfigurations(Map configuration) {
+
+    // TODO(ager): Get rid of this. This is for backwards
+    // compatibility with the python test scripts. They use system
+    // 'win32' for Windows.
+    if (configuration['system'] == 'windows') {
+      configuration['system'] = 'win32';
+    }
+
     // Expand the pseudo-values such as 'all'.
     if (configuration['architecture'] == 'all') {
       configuration['architecture'] = 'ia32,x64,simarm';