Add FluentResource

[zbraniecki]

This implements the first part of https://bugzilla.mozilla.org/show_bug.cgi?id=1455649 in fluent.js allowing for caching of parsed resources.

[stasm]

This looks good code-wise, but before approving the PR I'd like to learn your opinion on where the parsing logic should live. See #217 (comment) for @Pike's thoughts on this. I think I agree: it would likely make more sense for the parser to be part of FluentResource. Given that this is supposed to help the Gecko use-case, could you weigh on which approach is the best from the Gecko POV?

[stasm]

Please use this here rather than MessageContext to allow for subclassing.

[zbraniecki]

Using this throws on babel unfortunately here :(

[zbraniecki]

Given that this is supposed to help the Gecko use-case, could you weigh on which approach is the best from the Gecko POV?

I'm not strongly opinionated here. Happy to go either way.

Eventually there may be some complication when we try to transfer FluentResource between processes. I'm not sure how much hassle it adds to have methods defined on the data structure there, but we can cross that bridge when we come to it.

[zbraniecki]

Should I move the parser to FluentResource then, @stasm?

[stasm]

Eventually there may be some complication when we try to transfer FluentResource between processes. I'm not sure how much hassle it adds to have methods defined on the data structure there, but we can cross that bridge when we come to it.

My hope and the intent behind the extends Map design is that we will be able to "downgrade" to a regular Map in IPC scenarios, which the structured clone algorithm supports. This would happen automatically whenever a FluentResource instance is postMessaged.

Once its FTL is parsed, FluentResource doesn't really need any parsing capabilities. MessageContext.addResource can effectively take anything with an interface of a Map.

So yeah, let's move parsing to FluentResource, please. It's OK to keep it the code in parser.js and import it into resource.js. We don't need to expose parsing as a public method of FluentResource AFAICT.

[stasm]

Sweet, thanks!