Support paths which are encoded with base64 in http put devfs server code

Support paths which are encoded with base64 in http put devfs server code

- [x] Add a new header 'dev_fs_path_b64' which is used over 'dev_fs_path' when present.
- [x] 'dev_fs_path_b64' expects the path encoded as UTF8 and then BASE64.
- [x] Add unit test case.

BUG=
R=turnidge@google.com

Committed: https://github.com/dart-lang/sdk/commit/8b37da82ef98e2a57690e580d7335a49f07f0ca8

[Cutch, 2016-10-04 22:22:49]

Description was changed from

==========
Support paths which are encoded with base64 in http put devfs server code

BUG=
==========

to

==========
Support paths which are encoded with base64 in http put devfs server code

- [x] Add a new header 'dev_fs_path_b64' which is used over 'dev_fs_path' when
present.
- [x] 'dev_fs_path_b64' expects the path encoded as UTF8 and then BASE64.
- [x] Add unit test case.

BUG=
==========

[Cutch, 2016-10-04 22:22:49]

johnmccutchan@google.com changed reviewers:
+ turnidge@google.com

[turnidge, 2016-10-04 22:28:04]

lgtm

[Cutch, 2016-10-04 22:31:14]

Description was changed from

==========
Support paths which are encoded with base64 in http put devfs server code

- [x] Add a new header 'dev_fs_path_b64' which is used over 'dev_fs_path' when
present.
- [x] 'dev_fs_path_b64' expects the path encoded as UTF8 and then BASE64.
- [x] Add unit test case.

BUG=
==========

to

==========
Support paths which are encoded with base64 in http put devfs server code

- [x] Add a new header 'dev_fs_path_b64' which is used over 'dev_fs_path' when
present.
- [x] 'dev_fs_path_b64' expects the path encoded as UTF8 and then BASE64.
- [x] Add unit test case.

BUG=
R=turnidge@google.com

Committed:
https://github.com/dart-lang/sdk/commit/8b37da82ef98e2a57690e580d7335a49f07f0ca8
==========

[Cutch, 2016-10-04 22:31:16]

Committed patchset #3 (id:40001) manually as
8b37da82ef98e2a57690e580d7335a49f07f0ca8 (presubmit successful).