fix #1240

[Rich-Harris]

The solution in this PR doesn't work; in fact it highlights the fact that the current approach has fatal flaws. Leaving it here for posterity (along with the previously-failing test)

[codecov-io]

Codecov Report

Merging #1243 into master will decrease coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1243      +/-   ##
==========================================
- Coverage    91.9%   91.82%   -0.08%     
==========================================
  Files         126      126              
  Lines        4557     4700     +143     
  Branches     1486     1544      +58     
==========================================
+ Hits         4188     4316     +128     
- Misses        153      161       +8     
- Partials      216      223       +7

Impacted Files	Coverage Δ
src/generators/nodes/Component.ts	95.02% <100%> (+0.05%)	⬆️
src/generators/nodes/EachBlock.ts	98.11% <100%> (-0.02%)	⬇️
src/generators/dom/Block.ts	96.63% <100%> (-0.22%)	⬇️
src/parse/read/directives.ts	79.04% <0%> (-8.05%)	⬇️
src/parse/state/tag.ts	97.32% <0%> (+1.19%)	⬆️

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[Rich-Harris]

Have checked the original REPL example locally and it's fixed as of this PR. This feels like the right solution; now just need to stop passing around context values that aren't necessary (#1194)