Add Directory.systemTemp getter to replace createSystemTemp().

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';
 
 main() {
   group('Generate docs for', () {
     test('one simple file.', () {
-      var temporaryDir = Directory.createSystemTempSync('single_library_');
+      var temporaryDir = Directory.systemTemp.createTempSync('single_library_');
       var fileName = path.join(temporaryDir.path, 'temp.dart');
       var file = new File(fileName);
       file.writeAsStringSync('''
   library test;
   /**
    * Doc comment for class [A].
    * 
    * Multiline Test
    */
   /*
@@ skipping 101 matching lines @@
           expect(methodDocComment == 'test.A.doThis', isTrue);
           
           // Testing something with no reference
           var libraryDocComment = fixReference('foobar', libraryMirror,
               classMirror, methodMirror).children.first.text;
           expect(libraryDocComment == 'foobar', isTrue);
         })).whenComplete(() => temporaryDir.deleteSync(recursive: true));
     });
   });
 }