Fix incorrect rendering of export declarations in SystemJS

[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 #4201

Description

Due to an ordering issue, export declarations with an initializer that was a simplified expression would lead to incorrect results.

[github-actions]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#gh-4201-fix-system-declarations

or load it into the REPL:
https://rollupjs.org/repl/?pr=4202

[codecov]

Codecov Report

Merging #4202 (77033cc) into master (ffd5cad) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4202   +/-   ##
=======================================
  Coverage   98.37%   98.37%           
=======================================
  Files         202      202           
  Lines        7253     7253           
  Branches     2126     2126           
=======================================
  Hits         7135     7135           
  Misses         58       58           
  Partials       60       60           

Impacted Files	Coverage Δ
src/ast/nodes/VariableDeclaration.ts	98.05% <100.00%> (ø)

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