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Uses python AST parsing to ensure that the keys for swarming mixins are
sorted.

Change-Id: I450a683454903ad8a5a730a3abc3c9ef2c482f29
Reviewed-on: https://chromium-review.googlesource.com/1231832
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592271}