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

tools/testing/dart/configuration.dart

Index: tools/testing/dart/configuration.dart
diff --git a/tools/testing/dart/configuration.dart b/tools/testing/dart/configuration.dart
index dab39bf7e265627b4270b04bc4fa4f311f01d86b..58791fe31ad4e32dcd2e52560a9e1fbfdbe517c7 100644
--- a/tools/testing/dart/configuration.dart
+++ b/tools/testing/dart/configuration.dart
@@ -526,7 +526,7 @@ class Compiler {
         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];