Fix docgen Uri handling so it works on Windows.

pkg/docgen/test/single_library_test.dart

 library single_library_test;
 
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
 import 'package:unittest/unittest.dart';
 
 import '../lib/docgen.dart';
 
 const String DART_LIBRARY = '''
@@ skipping 28 matching lines @@
   }
 ''';
 
 main() {
   group('Generate docs for', () {
     test('one simple file.', () {
       var temporaryDir = Directory.systemTemp.createTempSync('single_library_');
       var fileName = path.join(temporaryDir.path, 'temp.dart');
       var file = new File(fileName);
       file.writeAsStringSync(DART_LIBRARY);
-      getMirrorSystem([fileName])
+      getMirrorSystem([new Uri.file(fileName)])
         .then(expectAsync1((mirrorSystem) {
           var testLibraryUri = new Uri(scheme: 'file',
               path: path.absolute(fileName));
           var library = generateLibrary(mirrorSystem.libraries[testLibraryUri]);
           expect(library is Library, isTrue);
 
           var classTypes = library.classes;
           expect(classTypes is ClassGroup, isTrue);
 
           var classes = [];
@@ skipping 67 matching lines @@
           expect(methodDocComment == 'test.A.doThis', isTrue);
 
           // Testing something with no reference
           var libraryDocComment = fixReference('foobar', libraryMirror,
               classMirror, methodMirror).text;
           expect(libraryDocComment == 'foobar', isTrue);
         })).whenComplete(() => temporaryDir.deleteSync(recursive: true));
     });
   });
 }