Remove one more use of DillTarget.read that broke compiler/tool/generate_kernel.

pkg/compiler/tool/generate_kernel_test.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.
+
+library compiler.tool.generate_kernel_test;
+
+import 'dart:io';
+import 'generate_kernel.dart' as m;
+import 'package:front_end/src/fasta/testing/patched_sdk_location.dart';
+
+main() async {
+  Directory dir;
+  try {
+    dir = Directory.systemTemp.createTempSync('generate_kernel_test');
+    var file = dir.absolute.uri.resolve('hi.dart');
+    new File.fromUri(file).writeAsStringSync("main() => print('hello world');");
+    var vmSdk = await computePatchedSdk();
+    var platformUri = vmSdk.resolve('../patched_dart2js_sdk/platform.dill');
+    await m.main(['--platform=${platformUri.toFilePath()}', file.toFilePath()]);
+  } finally {
+    dir.deleteSync(recursive: true);
+  }
+}