Delay computing file name sets until they are requested.
Delay computing file name sets until they are requested.

This makes Flutter repo analysis about 1 second faster.

R=1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com, devoncarew@google.com

Before: 74.8 74.8 73.3 72.7
After:  72.3 72.2 72.2 72.6
Change-Id: Iedc6a3d25a4de051b9207fbb6e549e7f3575a75a
Reviewed-on: https://dart-review.googlesource.com/51020
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>
Commit-Queue: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>

dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk