Update mutable objects and arrays properly

[Panya]

fixes #2290. I'm not sure that this is the right place to fix it, but all tests are green.

[codecov-io]

Codecov Report

Merging #2327 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2327   +/-   ##
=======================================
  Coverage   91.83%   91.83%           
=======================================
  Files           1        1           
  Lines          49       49           
=======================================
  Hits           45       45           
  Misses          4        4

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[Rich-Harris]

Thanks for the PR, and apologies for letting it sit for so long. Unfortunately this isn't the correct fix — it works for {obj.x} but fails for {identity(obj.x)} or any other non-member-expression expression that resolves to a mutable value. I've opened an alternative PR, and will close this in favour of that: #3394