Flush on write in FileByteStore.

Flush on write in FileByteStore.

We see (rarely, but still) situations when FileState attempts to read
a *.unlinked file, but the file is empty. We think that this happens
when the user's machine crashes, so files are created, but not fully
written to disk.

This change has its cost - increase of FileByteStore.put() CPU usage
from 7% to 10% on Mac.


R=paulberry@google.com, brianwilkerson@google.com
BUG=

Committed: https://github.com/dart-lang/sdk/commit/2d0d00061190d1df824628e7ec045ece27af7fd1

[Paul Berry, 2017-04-14 17:49:26]

lgtm

[scheglov, 2017-04-14 17:53:28]

Description was changed from

==========
Flush on write in FileByteStore.

We see (rarely, but still) situations when FileState attempts to read
a *.unlinked file, but the file is empty. We think that this happens
when the user's machine crashes, so files are created, but not fully
written to disk.

This change has its cost - increase of FileByteStore.put() CPU usage
from 7% to 10% on Mac.


R=brianwilkerson@google.com, paulberry@google.com
BUG=
==========

to

==========
Flush on write in FileByteStore.

We see (rarely, but still) situations when FileState attempts to read
a *.unlinked file, but the file is empty. We think that this happens
when the user's machine crashes, so files are created, but not fully
written to disk.

This change has its cost - increase of FileByteStore.put() CPU usage
from 7% to 10% on Mac.


R=paulberry@google.com, brianwilkerson@google.com
BUG=

Committed:
https://github.com/dart-lang/sdk/commit/2d0d00061190d1df824628e7ec045ece27af7fd1
==========

[scheglov, 2017-04-14 17:53:29]

Committed patchset #1 (id:1) manually as
2d0d00061190d1df824628e7ec045ece27af7fd1 (presubmit successful).

[Brian Wilkerson, 2017-04-17 17:29:09]

lgtm