dart:io | Add Link.targetSync on all platforms.

sdk/lib/_internal/compiler/implementation/lib/io_patch.dart

 // Copyright (c) 2013, 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.
 
 patch class _BufferUtils {
   patch static bool _isBuiltinList(List buffer) {
     throw new UnsupportedError("_isBuiltinList");
   }
 }
 
@@ skipping 51 matching lines @@
 patch class _File {
   patch static _exists(String path) {
     throw new UnsupportedError("File._exists");
   }
   patch static _create(String path) {
     throw new UnsupportedError("File._create");
   }
   patch static _createLink(String path, String target) {
     throw new UnsupportedError("File._createLink");
   }
+  patch static _linkTarget(String path) {
+    throw new UnsupportedError("File._linkTarget");
+  }
   patch static _delete(String path) {
     throw new UnsupportedError("File._delete");
   }
   patch static _directory(String path) {
     throw new UnsupportedError("File._directory");
   }
   patch static _lengthFromPath(String path) {
     throw new UnsupportedError("File._lengthFromPath");
   }
   patch static _lastModified(String path) {
@@ skipping 164 matching lines @@
 }
 
 patch class _Filter {
   patch static _Filter newZLibDeflateFilter(bool gzip, int level) {
     throw new UnsupportedError("newZLibDeflateFilter");
   }
   patch static _Filter newZLibInflateFilter() {
     throw new UnsupportedError("newZLibInflateFilter");
   }
 }