Analysis server's BazelWorkspace needs to support `dart_package` directives in BUILD files

[stereotype441]

The way to opt packages into NNBD in a Bazel workspace will be to include this directive in a BUILD file:

dart_package(
    non_nullable = True,
)

The analysis server needs to recognize this pattern and use it to determine whether a package is opted in to NNBD.

Note that BUILD files are auto-formatted so it's not necessary to parse the whole file to find this directive; a simple regexp should suffice.

[scheglov]

See also https://buganizer.corp.google.com/issues/162609651

[davidmorgan]

Thanks! A few notes:

It's true that the file is auto formatted, but some variation is allowed. These are all valid:

dart_package(non_nullable = True)

dart_package(
    non_nullable = True,
)

dart_package(
    # Preceding comment.
    non_nullable = True,  # Trailing comment.
)  # Last comment.

And: it's desirable that the analyzer react to changes to the BUILD file, so that adding non_nullable = True changes the analysis mode for the package as soon as the file is saved.

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/157474