Don't allow "none" as a runtime for dartk.

tools/testing/dart/test_runner.dart

Index: tools/testing/dart/test_runner.dart
diff --git a/tools/testing/dart/test_runner.dart b/tools/testing/dart/test_runner.dart
index 145e3ddf7299f30c00b83e5d5ba44c56a316f2ea..7549f073fcd6a491185934c2b39284b46afd862a 100644
--- a/tools/testing/dart/test_runner.dart
+++ b/tools/testing/dart/test_runner.dart
@@ -761,6 +761,9 @@ class TestCase extends UniqueObject {
   TestCase(this.displayName, this.commands, this.configuration,
       this.expectedOutcomes,
       {bool isNegative: false, TestInformation info}) {
+    // A test case should do something.
+    assert(commands.isNotEmpty);
+
     if (isNegative || displayName.contains("negative_test")) {
       _expectations |= IS_NEGATIVE;
     }