Add support for Flow declare export (default)

[jamiebuilds]

Q	A
Bug fix?	no
Breaking change?	no
New feature?	yes
Deprecations?	no
Spec compliancy?	sorta?
Tests added/pass?	yes
Fixed tickets	N/A
License	MIT

Adds support for the following Flow syntaxes:

declare module A { declare export default {}; }
declare module A { declare export default number; }
declare module A { declare export default var val: number; }
declare module A { declare export default function method(): number; }

declare module A { declare export var val: number; }
declare module A { declare export function method(): number; }

cc @samwgoldman

[codecov]

Codecov Report

Merging #441 into master will decrease coverage by 1.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #441      +/-   ##
==========================================
- Coverage   98.25%   97.02%   -1.24%     
==========================================
  Files          20       20              
  Lines        3504     3529      +25     
  Branches      927      932       +5     
==========================================
- Hits         3443     3424      -19     
- Misses         22       54      +32     
- Partials       39       51      +12

Flag	Coverage Δ
#babel	?
#babylon	97.02% <100%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
src/plugins/flow.js	97.3% <100%> (-0.89%)	⬇️
src/parser/expression.js	93.84% <0%> (-3.54%)	⬇️
src/tokenizer/state.js	97.22% <0%> (-2.78%)	⬇️
src/parser/index.js	97.29% <0%> (-2.71%)	⬇️
src/tokenizer/index.js	97.31% <0%> (-1.04%)	⬇️
src/parser/statement.js	98.15% <0%> (-0.93%)	⬇️

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[jamiebuilds]

I'm not a huge fan of how I did this (especially since node is potentially unused), but otherwise I'd be repeating a bunch of stuff.

[jamiebuilds]

Test failures don't seem to be a result of this PR

[jamiebuilds]

Note: The generated AST just copies what Flow has.

[hzoo]

Hey @thejameskyle! It looks like one or more of your builds have failed. I've copied the relevant info below to save you some time.

• Node vlts/* - linux
• Node vlts/* - linux

[danez]

Duplicate of #224