Support declare export statements

[danez]

Q	A
Patch: Bug Fix?	n
Major: Breaking Change?	n
Minor: New Feature?	y
Deprecations?
Spec Compliancy?
Tests Added/Pass?	y
Fixed Tickets
License	MIT
Doc PR
Dependency Changes

Counterpart of babel/babylon#224

[mention-bot]

@danez, thanks for your PR! By analyzing the history of the files in this pull request, we identified @loganfsmyth, @zpao and @samwgoldman to be potential reviewers.

[codecov]

Codecov Report

Merging #5589 into 7.0 will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              7.0    #5589      +/-   ##
==========================================
+ Coverage   85.09%   85.18%   +0.08%     
==========================================
  Files         284      284              
  Lines        9911     9953      +42     
  Branches     2768     2776       +8     
==========================================
+ Hits         8434     8478      +44     
+ Misses        976      974       -2     
  Partials      501      501

Impacted Files	Coverage Δ
packages/babel-types/src/definitions/flow.js	100% <100%> (ø)	⬆️
packages/babel-generator/src/generators/flow.js	97.79% <100%> (+0.38%)	⬆️
packages/babel-traverse/src/path/context.js	85.34% <0%> (ø)	⬆️
...bel-plugin-transform-es2015-classes/src/vanilla.js	90.63% <0%> (ø)	⬆️
packages/babel-traverse/src/path/modification.js	73.07% <0%> (+0.96%)	⬆️
packages/babel-helper-call-delegate/src/index.js	68% <0%> (+4%)	⬆️

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