Add file modes for opening a file write only

Add file modes for opening a file write only

Added two new file modes WRITE_ONLY and WRITE_ONLY_APPEND. The current
file modes WRITE and WRITE_APPEND are kept as the read/write mode with
no change. Kept the same overall handling of create/truncate/append
semantics for the new file modes.

R=ager@google.com, ahe@google.com
BUG=

Committed: https://github.com/dart-lang/sdk/commit/eaeecf2ed13ba6c7fbfd653c3c592974a7120960

[Søren Gjesse, 2015-06-18 11:34:39]

On 2015/06/18 11:34:22, Søren Gjesse wrote:

This is Linux only for now.

[Søren Gjesse, 2015-06-18 11:41:23]

Now with Mac, Android and Windows.

[Mads Ager (google), 2015-06-18 12:04:48]

LGTM, thanks Soren! :)

https://codereview.chromium.org/1193653002/diff/20001/sdk/lib/io/file.dart
File sdk/lib/io/file.dart (right):

https://codereview.chromium.org/1193653002/diff/20001/sdk/lib/io/file.dart#ne...
sdk/lib/io/file.dart:42: const WRITE_ONLY = FileMode.WRITE_ONLY;
You should duplicate the doc comments here.

[Søren Gjesse, 2015-06-18 13:19:33]

Committed patchset #3 (id:40001) manually as
eaeecf2ed13ba6c7fbfd653c3c592974a7120960 (presubmit successful).

[ahe, 2015-06-18 14:50:42]

Thank you!

[kevmoo, 2015-06-30 21:31:05]

On 2015/06/18 14:50:42, ahe wrote:
> Thank you!

Please update CHANGELOG.md for API changes.