Fix file error tests on Windows
Fix file error tests on Windows

* Always close files before deleting the temporary test directory
* Truncate on read only fils only fails if the file is not empty

TBR=ager@google.com

BUG=none
TEST=tests/standalone/src/io/FileErrorTest.dart

Review URL: https://chromiumcodereview.appspot.com//9855012

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@5814 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk