Fix ASI bug in react-hot-loader transform

[alangpierce]

The specific pattern export default function() {} was transformed in such a
way that the parenthesized block afterword looked like a function invocation. To
avoid this, we can start the snippet with a semicolon.

[codecov-io]

Codecov Report

Merging #384 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #384      +/-   ##
=========================================
- Coverage   79.86%   79.8%   -0.06%     
=========================================
  Files          45      45              
  Lines        5159    4937     -222     
  Branches     1147    1147              
=========================================
- Hits         4120    3940     -180     
+ Misses        739     699      -40     
+ Partials      300     298       -2

Impacted Files	Coverage Δ
src/transformers/ReactHotLoaderTransformer.ts	100% <ø> (ø)	⬆️
src/util/getNonTypeIdentifiers.ts	75% <0%> (-25%)	⬇️
src/parser/index.ts	77.77% <0%> (-7.94%)	⬇️
src/index.ts	82.85% <0%> (-6.51%)	⬇️
src/computeSourceMap.ts	83.33% <0%> (-5.56%)	⬇️
src/parser/plugins/types.ts	90% <0%> (-4.45%)	⬇️
src/util/getTSImportedNames.ts	81.57% <0%> (-3.79%)	⬇️
src/parser/tokenizer/readWord.ts	82.35% <0%> (-3.02%)	⬇️
src/parser/traverser/util.ts	86.2% <0%> (-2.17%)	⬇️
src/parser/traverser/base.ts	95.45% <0%> (-1.69%)	⬇️
... and 14 more

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