Move close-paren to before arrow when removing arrow return types

[alangpierce]

Fixes #391

It's a bit ugly, but we now do a lookahead whenever we see a ): with the :
as a type token. If the next non-type is an arrow, we move the ) to just
before the arrow. This seems to work and seems to agree with
processBalancedCode.

I also added a new type of test that actually executes the resulting code and
asserts the output value from the code, which will probably be useful in the
future as well.

[codecov-io]

Codecov Report

Merging #393 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #393     +/-   ##
=========================================
+ Coverage   79.81%   79.91%   +0.1%     
=========================================
  Files          45       45             
  Lines        4939     5173    +234     
  Branches     1148     1152      +4     
=========================================
+ Hits         3942     4134    +192     
- Misses        699      739     +40     
- Partials      298      300      +2

Impacted Files	Coverage Δ
src/transformers/FlowTransformer.ts	100% <100%> (ø)	⬆️
src/transformers/TypeScriptTransformer.ts	95% <100%> (ø)	⬆️
src/transformers/RootTransformer.ts	92.48% <100%> (+0.51%)	⬆️
src/util/formatTokens.ts	70.58% <0%> (-10.06%)	⬇️
src/parser/util/identifier.ts	90.9% <0%> (-4.1%)	⬇️
src/parser/plugins/jsx/index.ts	87.33% <0%> (-1.88%)	⬇️
src/parser/plugins/flow.ts	62.21% <0%> (-1.59%)	⬇️
src/util/getClassInfo.ts	85.12% <0%> (-1.21%)	⬇️
src/identifyShadowedGlobals.ts	87.8% <0%> (-1.09%)	⬇️
src/parser/traverser/statement.ts	80.28% <0%> (-0.77%)	⬇️
... and 16 more

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