this._changeHandlers was undefined when calling cancel()

[GarethOates]

This change fixes Issue #1177 where the scope of the cancel() function was not bound correctly and thus when attempting to call cancel() a type error of this._changeHandlers is undefined was created.

[codecov-io]

Codecov Report

Merging #1178 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1178   +/-   ##
=======================================
  Coverage   91.75%   91.75%           
=======================================
  Files         127      127           
  Lines        4572     4572           
  Branches     1495     1495           
=======================================
  Hits         4195     4195           
  Misses        159      159           
  Partials      218      218

Impacted Files	Coverage Δ
src/parse/read/directives.ts	73.43% <0%> (ø)	⬆️
src/parse/read/expression.ts	100% <0%> (ø)	⬆️

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[PaulBGD]

Could you create a test that fails without this change?

[GarethOates]

Added a test that fails without this change.

[Conduitry]

Anyone know what the performance difference is between calling .bind(this) and saving a reference with var self = this; outside the function and using self... instead of this...?

[Rich-Harris]

Thanks @GarethOates!

Anyone know what the performance difference is

I put together a lil ESBench that suggests saving a reference is cheaper, which I suppose makes sense (maybe the difference is less noticeable if you're calling the bound function many times, but cancel() is only called once) — I tweaked the PR accordingly in 957179f.