Avoid false positive when detecting if a class has a superclass

[alangpierce]

Fixes #397

I'm fairly sure that all false positives are type tokens, so we can just make
sure that we only look for non-type extends tokens.

[codecov-io]

Codecov Report

Merging #399 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
+ Coverage   79.86%   79.95%   +0.09%     
==========================================
  Files          45       45              
  Lines        4951     5173     +222     
  Branches     1152     1152              
==========================================
+ Hits         3954     4136     +182     
- Misses        699      738      +39     
- Partials      298      299       +1

Impacted Files	Coverage Δ
src/util/getClassInfo.ts	85.12% <100%> (-1.21%)	⬇️
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/identifyShadowedGlobals.ts	87.8% <0%> (-1.09%)	⬇️
src/parser/traverser/statement.ts	80.28% <0%> (-0.77%)	⬇️
src/util/isIdentifier.ts	100% <0%> (ø)	⬆️
src/parser/util/whitespace.ts	100% <0%> (ø)	⬆️
... and 15 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c5fa8c7...568c77b. Read the comment docs.