Hoist toSequenceExpression's convert helper (#5693)

[jridgewell]

Q	A
Patch: Bug Fix?	Yes
Major: Breaking Change?	No
Minor: New Feature?	No?
Deprecations?	No
Spec Compliancy?	No
Tests Added/Pass?	Lots
Fixed Tickets	Fixes #5080, Fixes #4224
License	MIT
Doc PR
Dependency Changes

Ports #5693 to 7.0.

[mention-bot]

@jridgewell, thanks for your PR! By analyzing the history of the files in this pull request, we identified @hzoo, @boopathi and @zertosh to be potential reviewers.

[codecov]

Codecov Report

Merging #5753 into 7.0 will decrease coverage by 0.01%.
The diff coverage is 97.43%.

@@            Coverage Diff             @@
##              7.0    #5753      +/-   ##
==========================================
- Coverage   84.61%   84.59%   -0.02%     
==========================================
  Files         282      282              
  Lines        9855     9857       +2     
  Branches     2766     2769       +3     
==========================================
  Hits         8339     8339              
- Misses        998     1003       +5     
+ Partials      518      515       -3

Impacted Files	Coverage Δ
packages/babel-types/src/converters.js	95.23% <97.43%> (+2.82%)	⬆️
packages/babel-helper-call-delegate/src/index.js	64% <0%> (-4%)	⬇️
packages/babel-traverse/src/path/modification.js	72.11% <0%> (-0.97%)	⬇️
packages/babel-traverse/src/visitors.js	85.71% <0%> (-0.96%)	⬇️
packages/babel-traverse/src/path/context.js	85.34% <0%> (-0.87%)	⬇️
...bel-plugin-transform-es2015-classes/src/vanilla.js	90.12% <0%> (-0.86%)	⬇️

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