ES2015: Modules: export declarations (partial)

[gasi]

This is a follow-up to #79.

After reading the spec more and approaches such as Shift AST, I concluded we want to introduce Module as a top-level abstraction next to Program (and later replace Program with Script):

The top-level node returned from any successful parse is named Script, not Program, to ease upgrade to ECMAScript 6. ECMAScript 6 parsers need two modes: one mode that produces a Script and one mode that produces a Module. Modules allow import/export declarations at the top level and are always in strict mode.

— https://blog.shapesecurity.com/2015/01/06/a-technical-comparison-of-the-shift-and-spidermonkey-ast-formats/

This makes it less likely for invalid programs to inhabit the AST, e.g. export in a Program (later: Script). I like their approach:

A good AST format…

• minimizes the number of inhabitants that do not represent a program.
• is at least partially homogenous to allow for a simple and efficient visitor.
• does not impede moving, copying, or replacing subtrees.
• discourages duplication in code that operates on it.

I found out about Shift AST in connection with this project somewhere (can’t find the reference anymore). What’s the approach for the new AST? Would it make sense to follow Shift AST spec if possible? Might make for some nice potential interop and learning opportunities.

Update: Found the reference: #41 (comment)

[erikd]

Manually squashed and merged. Thanks!

[gasi]

@erikd Just got back from vacation. Thanks for the review and merging 👍