`_IndexAssembler.assemble`: optimize using `toList(growable: false)`

[gmpassos]

The assemble method is a hotspot for List generation, and a fixed-length list is faster to generate.

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• [✅] I’ve reviewed the contributor guide and applied the relevant portions to this PR.

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[mraleph]

@gmpassos @bwilkerson what is the status of this change?

[gmpassos]

The last update on https://dart-review.googlesource.com/c/sdk/+/381780 is:

This CL has passed the run

[copybara-service[bot]]

https://dart-review.googlesource.com/c/sdk/+/381780 has been updated with the latest commits from this pull request.

[copybara-service[bot]]

https://dart-review.googlesource.com/c/sdk/+/381780 has been updated with the latest commits from this pull request.

[mraleph]

@bwilkerson @scheglov same here, please make a call and either accept (+1 & +2 the CL) or reject the PR. Thanks!