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

tools/testing/dart/configuration.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'compiler_configuration.dart';
 import 'http_server.dart';
@@ skipping 508 matching lines @@
           Runtime.ie11,
           Runtime.opera,
           Runtime.chromeOnAndroid,
           Runtime.safariMobileSim
         ];
 
       case Compiler.dart2analyzer:
         return const [Runtime.none];
       case Compiler.appJit:
       case Compiler.dartk:
-        return const [Runtime.vm, Runtime.selfCheck, Runtime.none];
+        return const [Runtime.vm, Runtime.selfCheck];
       case Compiler.precompiler:
       case Compiler.dartkp:
         return const [Runtime.dartPrecompiled];
       case Compiler.none:
         return const [
           Runtime.vm,
           Runtime.flutter,
           Runtime.drt,
           Runtime.dartium,
           Runtime.contentShellOnAndroid,
@@ skipping 188 matching lines @@
 
       case macos:
         return 'xcodebuild/';
     }
 
     throw "unreachable";
   }
 
   String toString() => "System($name)";
 }