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

tools/testing/dart/test_runner.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Classes and methods for executing tests.
  *
  * This module includes:
  * - Managing parallel execution of tests, including timeout checks.
  * - Evaluating the output of each test as pass/fail/crash/timeout.
@@ skipping 743 matching lines @@
 
   Configuration configuration;
   String displayName;
   int _expectations = 0;
   int hash = 0;
   Set<Expectation> expectedOutcomes;
 
   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;
     }
     if (info != null) {
       _setExpectations(info);
       hash =
           info.originTestPath.relativeTo(TestUtils.dartDir).toString().hashCode;
     }
   }
 
@@ skipping 2457 matching lines @@
     }
   }
 
   void eventAllTestsDone() {
     for (var listener in _eventListener) {
       listener.allDone();
     }
     _allDone();
   }
 }