Avoid flaking in message_extraction tests by doing everything in a temp directory

pkg/intl/test/message_extraction/failed_extraction_test.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 failed_extraction_test;
+
 import "message_extraction_test.dart";
 import "dart:io";
 import "package:unittest/unittest.dart";
 
 main() {
-  run(null, ["extract_to_json.dart",
-      "sample_with_messages.dart", "part_of_sample_with_messages.dart"])
-    .then((ProcessResult result) {
-      expect(result.exitCode, 0);
+  test("Expect warnings but successful extraction", () {
+    runTestWithWarnings(warningsAreErrors: false, expectedExitCode: 0);
   });
-  run(null, ["extract_to_json.dart", "--warnings-are-errors",
-      "sample_with_messages.dart", "part_of_sample_with_messages.dart"])
-    .then((ProcessResult result) {
-      expect(result.exitCode, 1);
-    });
-}
+}
+
+void runTestWithWarnings({bool warningsAreErrors, int expectedExitCode}) {
+
+  void verify(ProcessResult result) {
+    try {
+      expect(result.exitCode, expectedExitCode);
+    } finally {
+      deleteGeneratedFiles();
+    }
+  }
+
+  copyFilesToTempDirectory();
+  var program = asTestDirPath("extract_to_json.dart");
+  var args = ["--output-dir=$tempDir"];
+  if (warningsAreErrors) {
+    args.add('--warnings-are-errors');
+  }
+  var files = [asTempDirPath("sample_with_messages.dart"), asTempDirPath(
+      "part_of_sample_with_messages.dart"),];
+  var allArgs = [program]
+      ..addAll(args)
+      ..addAll(files);
+  var callback = expectAsync(verify);
+  run(null, allArgs).then(callback);
+}