This patch lands the core of the new localization API:
* Parser for the Fluent syntax called "FTL"
* Resolver for the Fluent logic
* MessageContext class which is a central class for storing and operating
on Fluent messages.
MozReview-Commit-ID: E7nKGsLOCJe
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extra : rebase_source : 9bb4c7b155c8b9bd8923534b69f643a6e85bb8af