Fix parens issues with exponentiation in generator

[existentialism]

Q	A
Patch: Bug Fix?	yes
Major: Breaking Change?	no
Minor: New Feature?	no
Deprecations?	no
Spec Compliancy?	no
Tests Added/Pass?	yes/yes
Fixed Tickets	Fixes #5829
License	MIT
Doc PR
Dependency Changes

/cc: @bakkot

[codecov]

Codecov Report

Merging #5830 into 7.0 will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              7.0    #5830      +/-   ##
==========================================
+ Coverage   85.09%   85.14%   +0.04%     
==========================================
  Files         284      284              
  Lines        9911     9916       +5     
  Branches     2768     2772       +4     
==========================================
+ Hits         8434     8443       +9     
+ Misses        976      972       -4     
  Partials      501      501

Impacted Files	Coverage Δ
packages/babel-generator/src/node/parentheses.js	97% <100%> (+0.15%)	⬆️
...bel-plugin-transform-es2015-classes/src/vanilla.js	91.06% <0%> (+0.42%)	⬆️
packages/babel-traverse/src/path/context.js	86.2% <0%> (+0.86%)	⬆️
packages/babel-traverse/src/path/modification.js	73.07% <0%> (+0.96%)	⬆️
packages/babel-helper-call-delegate/src/index.js	68% <0%> (+4%)	⬆️

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[hzoo]

cc @rauschma

[rauschma]

Just in case – all of these are syntax errors:

+a ** b ** c;
+(a ** b) ** c;
+a.b ** c;
+(-a) ** b;
+a ** -b;
+(a * b) ** c;
+a ** (b * c);

Source: packages/babel-generator/test/fixtures/parentheses/exponentiation/actual.js

(If these are not already negative checks then they should probably become ones.)

[hzoo]

For syntax errors I believe we would just rely on the parser, don't think we need to duplicate because it would just be the same error

[existentialism]

I'll double check the exponentiation tests in babylon (though I think most of the error cases are there)