Fix Flow bug where implements was assigned to the wrong token

[alangpierce]

Fixes #407

[codecov-io]

Codecov Report

Merging #409 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #409      +/-   ##
==========================================
+ Coverage   80.03%   80.05%   +0.01%     
==========================================
  Files          46       46              
  Lines        5184     5184              
  Branches     1155     1155              
==========================================
+ Hits         4149     4150       +1     
  Misses        737      737              
+ Partials      298      297       -1

Impacted Files	Coverage Δ
src/parser/plugins/flow.ts	62.21% <100%> (ø)	⬆️
src/parser/traverser/statement.ts	80.46% <0%> (+0.17%)	⬆️

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