Provide more data on failure

pkg/analyzer/test/generated/java_io_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.
 
 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 
 library engine.java_io_test;
 
 import 'dart:io';
 
 import 'package:analyzer/src/generated/java_io.dart';
 import 'package:unittest/unittest.dart';
 
 import '../utils.dart';
 
 main() {
   initializeTestEnvironment();
   group('JavaFile', () {
     group('toURI', () {
       test('forAbsolute', () {
         var tempDir = Directory.systemTemp.createTempSync('java_io_test');
         try {
           String tempPath = JavaFile.pathContext
               .normalize(JavaFile.pathContext.absolute(tempDir.path));
           String path = JavaFile.pathContext.join(tempPath, 'foo.dart');
           // we use an absolute path
-          expect(JavaFile.pathContext.isAbsolute(path), isTrue);
+          expect(JavaFile.pathContext.isAbsolute(path), isTrue, reason: '"$path" is not absolute');
           // test that toURI() returns an absolute URI
           Uri uri = new JavaFile(path).toURI();
           expect(uri.isAbsolute, isTrue);
           expect(uri.scheme, 'file');
         } finally {
           tempDir.deleteSync(recursive: true);
         }
       });
       test('forRelative', () {
         var tempDir = Directory.systemTemp.createTempSync('java_io_test');
         try {
           String tempPath = JavaFile.pathContext
               .normalize(JavaFile.pathContext.absolute(tempDir.path));
           String path = JavaFile.pathContext.join(tempPath, 'foo.dart');
-          expect(JavaFile.pathContext.isAbsolute(path), isTrue);
+          expect(JavaFile.pathContext.isAbsolute(path), isTrue, reason: '"$path" is not absolute');
           // prepare a relative path
           // We should not check that "relPath" is actually relative -
           // it may be not on Windows, if "temp" is on other disk.
           String relPath = JavaFile.pathContext.relative(path);
           // test that toURI() returns an absolute URI
           Uri uri = new JavaFile(relPath).toURI();
           expect(uri.isAbsolute, isTrue);
           expect(uri.scheme, 'file');
         } finally {
           tempDir.deleteSync(recursive: true);
         }
       });
     });
   });
 }