Change the behavior of open on files to not truncate by default

tools/testing/dart/multitest.dart

 // Copyright (c) 2011, 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("multitest");
 
 // Multitests are Dart test scripts containing lines of the form
 // " [some dart code] /// [key]: [error type]"
 //
 // For each key in the file, a new test file is made containing all
@@ skipping 118 matching lines @@
   String directory = CreateMultitestDirectory(outputDir, testDir);
   String pathSeparator = new Platform().pathSeparator();
   int start = filename.lastIndexOf(pathSeparator) + 1;
   int end = filename.indexOf('.dart', start);
   String baseFilename = filename.substring(start, end);
   for (String key in tests.getKeys()) {
     final String filename = '$directory/${baseFilename}_$key.dart';
     final File file = new File(filename);
 
     file.createSync();
-    RandomAccessFile openedFile = file.openSync(writable: true);
+    RandomAccessFile openedFile = file.openSync(FileMode.WRITE);
     var bytes = tests[key].charCodes();
     openedFile.writeListSync(bytes, 0, bytes.length);
     openedFile.closeSync();
     var outcome = outcomes[key];
     bool enableFatalTypeErrors = outcome.contains('static type error');
     bool isNegative = (outcome.contains('compile-time error') ||
                        outcome.contains('runtime error'));
     bool isNegativeIfChecked = outcome.contains('dynamic type error');
     doTest(filename,
            isNegative,
@@ skipping 13 matching lines @@
   var split = testDir.split('/');
   var lastComponent = split.removeLast();
   Expect.isTrue(lastComponent == 'src');
   String path = '${parentDir.path}/${split.last()}';
   Directory dir = new Directory(path);
   if (!dir.existsSync()) {
     dir.createSync();
   }
   return path;
 }