Factor out some common logic from the launchers.

test/runner/browser/dartium_test.dart

 // Copyright (c) 2015, 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.
 
 @TestOn("vm")
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
@@ skipping 138 matching lines @@
       await dartium.close();
     } finally {
       server.close();
     }
   });
 
   test("reports an error in onExit", () {
     var dartium = new Dartium("http://dart-lang.org",
         executable: "_does_not_exist");
     expect(dartium.onExit, throwsA(isApplicationException(startsWith(
-        "Failed to start Dartium: $noSuchFileMessage"))));
+        "Failed to run Dartium: $noSuchFileMessage"))));
   });
 
   test("can run successful tests", () {
     new File(p.join(_sandbox, "test.dart")).writeAsStringSync("""
 import 'package:test/test.dart';
 
 void main() {
   test("success", () {});
 }
 """);
@@ skipping 12 matching lines @@
 }
 """);
 
     var result = _runTest(["-p", "dartium", "test.dart"]);
     expect(result.exitCode, equals(1));
   });
 }
 
 ProcessResult _runTest(List<String> args) =>
     runTest(args, workingDirectory: _sandbox);