Do not partially tree-shake unused declarations in for loops

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:
Resolves #3941

Description

This is a fix for an issue introduced by partial declaration tree-shaking in for-loops. The simple solution here is not to tree-shake declarations in the first argument of a for-loop. A more involved solution would be to always include ids here, but I was not sure if it was worth the effort.

[rollup-bot]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#gh-3941-unused-for-loop-declaration

or load it into the REPL:
https://rollupjs.org/repl/?circleci=14111

[codecov]

Codecov Report

Merging #3943 (a517adc) into master (363e15d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3943   +/-   ##
=======================================
  Coverage   97.18%   97.18%           
=======================================
  Files         188      188           
  Lines        6681     6684    +3     
  Branches     1944     1946    +2     
=======================================
+ Hits         6493     6496    +3     
  Misses         99       99           
  Partials       89       89           

Impacted Files	Coverage Δ
src/ast/nodes/ForStatement.ts	100.00% <100.00%> (ø)
src/ast/nodes/VariableDeclaration.ts	97.97% <100.00%> (+0.02%)	⬆️
src/ast/nodes/VariableDeclarator.ts	100.00% <100.00%> (ø)

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