Use getDriverFor() and check for null.
Use getDriverFor() and check for null.

I see that sometimes we get a watcher event that AndroidManifest.xml
was changed, and we don't have the corresponding analysis context.
I suspect that these events are buffered, so we quickly first get
an event that forces us to re-create analysis contexts, and then
the event about AndroidManifest.xml, and if the new set of analysis
contexts does not include the AndroidManifest.xml, we get null.

Change-Id: Ie0bcfbb8f7baa6b6335bf6326f2ea2b7093a1b67
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/188741
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>
Commit-Queue: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk