Use the VM under test, not the prebuilt binary

dart/tools/testing/dart/test_suite.dart

Index: dart/tools/testing/dart/test_suite.dart
diff --git a/dart/tools/testing/dart/test_suite.dart b/dart/tools/testing/dart/test_suite.dart
index 15874b00be1184a576992319b8e551e416d82a12..a935b2c5ee05d2eb5674af94b35f789d619cf575 100644
--- a/dart/tools/testing/dart/test_suite.dart
+++ b/dart/tools/testing/dart/test_suite.dart
@@ -682,10 +682,10 @@ class StandardTestSuite implements TestSuite {
       var extraCommand = optionsFromFile['extraCommand'];
       if (extraCommand != null) {
         var args = optionsFromFile['extraCommandArgs'];
-        // As a special case, a command of "dart" should run with the same
-        // dart executable that we are using.
+        // As a special case, a command of "dart" should run with the
+        // dart VM that we are testing.
         if (extraCommand == 'dart') {
-          extraCommand = new Options().executable;
+          extraCommand = TestUtils.vmFileName(configuration),
         }
         args= args.map((arg)=>arg.replaceAll(@"$dartDir", dartDir.toString()));
         commands.add(new Command(extraCommand, args));