Fix null reference issues in bazel.dart.

Fix null reference issues in bazel.dart.

There were two issues:

- We were trying to use `parent` before null checking it.

- In the event that no workspace root could be found, we were returning
  `null` from the BazelWorkspace factory constructor.  This CL changes
  the behavior so that we return a BazelWorkspace that has no special
  bin or genfiles behaviors.

R=scheglov@google.com

Committed: https://github.com/dart-lang/sdk/commit/f9c819057bb5fda598a33d97ebf9418f36cd0932

[Paul Berry, 2016-10-06 15:51:27]

paulberry@google.com changed reviewers:
+ brianwilkerson@google.com, scheglov@google.com

[scheglov, 2016-10-06 15:53:29]

LGTM

[Brian Wilkerson, 2016-10-06 15:55:02]

> ... we return a BazelWorkspace that has no special bin or genfiles behaviors.

What is the user case for this change?

[Paul Berry, 2016-10-06 21:21:23]

Description was changed from

==========
Fix null reference issues in bazel.dart.

There were two issues:

- We were trying to use `parent` before null checking it.

- In the event that no workspace root could be found, we were returning
  `null` from the BazelWorkspace factory constructor.  This CL changes
  the behavior so that we return a BazelWorkspace that has no special
  bin or genfiles behaviors.
==========

to

==========
Fix null reference issues in bazel.dart.

There were two issues:

- We were trying to use `parent` before null checking it.

- In the event that no workspace root could be found, we were returning
  `null` from the BazelWorkspace factory constructor.  This CL changes
  the behavior so that we return a BazelWorkspace that has no special
  bin or genfiles behaviors.

R=scheglov@google.com

Committed:
https://github.com/dart-lang/sdk/commit/f9c819057bb5fda598a33d97ebf9418f36cd0932
==========

[Paul Berry, 2016-10-06 21:21:24]

Committed patchset #2 (id:20001) manually as
f9c819057bb5fda598a33d97ebf9418f36cd0932 (presubmit successful).