Make finalize() async and await adding imports, if any.

Make finalize() async and await adding imports, if any.

There were two flaky tests for assists because sometimes we managed
to execute adding imports before performing checks in the test. And
in this case there was additional (unexpected) empty file edit for
the library file (the only expected file edit was for the part).

R=brianwilkerson@google.com
BUG=

Committed: https://github.com/dart-lang/sdk/commit/dc605662936ecfa382d3a3278efa7e5df4d7bc5a

[Brian Wilkerson, 2017-08-25 04:02:08]

lgtm

[scheglov, 2017-08-25 04:07:41]

Description was changed from

==========
Make finalize() async and await adding imports, if any.

There were two flaky tests for assists because sometimes we managed
to execute adding imports before performing checks in the test. And
in this case there was additional (unexpected) empty file edit for
the library file (the only expected file edit was for the part).

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

to

==========
Make finalize() async and await adding imports, if any.

There were two flaky tests for assists because sometimes we managed
to execute adding imports before performing checks in the test. And
in this case there was additional (unexpected) empty file edit for
the library file (the only expected file edit was for the part).

R=brianwilkerson@google.com
BUG=

Committed:
https://github.com/dart-lang/sdk/commit/dc605662936ecfa382d3a3278efa7e5df4d7bc5a
==========

[scheglov, 2017-08-25 04:07:43]

Committed patchset #1 (id:1) manually as
dc605662936ecfa382d3a3278efa7e5df4d7bc5a (presubmit successful).