The existing CloudDnsWriter code uses ImmutableMap.Builder to construct the
map of DNS records to update. This has been seen to fail on alpha, presumably
in a cases where host records and domain records produce duplicate updates for
a host.
Convert the Builder to a HashMap, allowing us to safely overwrite existing
records in the case of duplicates.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170103421