This is useful for processing purely "local" DEPS files, with a command-line like:
  gclient revinfo --output-json /tmp/out \
    --spec 'solutions=[{"name": ".", "deps_file": "myDEPS", "url": None}]'

This is specifically to fix the "revinfo --output-json" case, but it
also cleans up some errors in the 'flattening' test cases. Note that
None values already work when using 'revinfo' with stdout output, it's
just the json output that complains.

BUG=825063

Change-Id: If5f69ba51cb9d0aa0f2b48a2f9b4ed8706b4c738
Reviewed-on: https://chromium-review.googlesource.com/989022
Commit-Queue: Michael Moss <mmoss@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>