Make special case for class property initializers in shadow-functions

[motiz88]

Q	A
Bug fix?	yes
Breaking change?	no
New feature?	no
Deprecations?	no
Spec compliancy?	yes
Tests added/pass?	yes
Fixed tickets	#4337 (partially), #4230 (needs verification)
License	MIT

This modifies shadow-functions to leave references to this and arguments unmodified in the context of a class property's value expression. The effect is akin to introducing an implicit scope around each such initializer, and is in line with the current proposal AFAICT.

All this applies only when emitting code with the class properties syntax and not running it through transform-class-properties - which is the only way such syntax can reach shadow-functions. The transform itself is correct and unchanged.

[codecov-io]

Current coverage is 88.74% (diff: 100%)

Merging #4502 into master will increase coverage by <.01%

@@             master      #4502   diff @@
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  Files           195        195          
  Lines         13773      13776     +3   
  Methods        1425       1425          
  Messages          0          0          
  Branches       3171       3173     +2   
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+ Hits          12223      12226     +3   
  Misses         1550       1550          
  Partials          0          0          

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[loganfsmyth]

Rather than using this flag, which took me a minute to process, could we perhaps do

let fnPath = path.find(function (innerPath) { // rename argument
    if (innerPath.parentPath && innerPath.parentPath.isClassProperty() && innerPath.key === "value") {
        return true;
    }
    if (path === innerPath) return false;

[motiz88]

Done.