Removing lingering GopherJS support debt

[slimsag]

This removes some remaining / lingering GopherJS support debt - which exists primarily due to supporting older GopherJS versions. #264

Fixes:

• Go 1.14+ requirement not properly type-checked (GopherJS legacy debt) #266
• incorrect-looking // +build line #261

[codecov-commenter]

Codecov Report

Merging #267 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #267   +/-   ##
=======================================
  Coverage   59.55%   59.55%           
=======================================
  Files           4        4           
  Lines         675      675           
=======================================
  Hits          402      402           
  Misses        215      215           
  Partials       58       58           

Impacted Files	Coverage Δ
dom_native.go	57.89% <ø> (ø)

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