Remove dead code with analyzer warning in test scripts

tools/testing/dart/runtime_configuration.dart

 // Copyright (c) 2014, 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.
 
 library runtime_configuration;
 
-import 'dart:io' show Platform;
-
 import 'compiler_configuration.dart' show CommandArtifact;
 
 // TODO(ahe): Remove this import, we can precompute all the values required
 // from TestSuite once the refactoring is complete.
 import 'test_suite.dart' show TestSuite;
 
 import 'test_runner.dart' show Command, CommandBuilder;
 
 // TODO(ahe): I expect this class will become abstract very soon.
 class RuntimeConfiguration {
@@ skipping 26 matching lines @@
 
       case 'd8':
         return new D8RuntimeConfiguration();
 
       case 'none':
         return new NoneRuntimeConfiguration();
 
       case 'vm':
         return new StandaloneDartRuntimeConfiguration();
 
-      case 'dart_app':
-        return new DartAppRuntimeConfiguration(useBlobs: useBlobs);
-
       case 'dart_precompiled':
         if (configuration['system'] == 'android') {
           return new DartPrecompiledAdbRuntimeConfiguration(useBlobs: useBlobs);
         }
         return new DartPrecompiledRuntimeConfiguration(useBlobs: useBlobs);
 
       case 'drt':
         return new DrtRuntimeConfiguration();
 
       default:
@@ skipping 230 matching lines @@
 class DummyRuntimeConfiguration extends DartVmRuntimeConfiguration {
   List<Command> computeRuntimeCommands(
       TestSuite suite,
       CommandBuilder commandBuilder,
       CommandArtifact artifact,
       List<String> arguments,
       Map<String, String> environmentOverrides) {
     throw "Unimplemented runtime '$runtimeType'";
   }
 }