Use dart2wasm app.support.js expression with `js-string` builtin requirement by mkustermann · Pull Request #183835 · flutter/flutter

mkustermann · 2026-03-18T13:26:47Z

This changes flutter web's detection of wasm compatibility to be the expression emitted by dart2wasm.

The expression was generated by passing --require-js-string-builtin to the dart2wasm compiler. Newer versions of browsers all support the js-string builtins nowadays. Apps running on browsers without js-string builtin support will not run in a performant way, so we'd not want to run the wasm version on them anyway.

This is a preparation for dart2wasm to require the js-string builtin by default (i.e. we'll remove the --require-js-string-builtin flag and also remove the polyfill for js-string builtins). See [0]

[0] https://dart-review.googlesource.com/c/sdk/+/488840

For reference of Safari testing on Dart:

cl/347283, cl/348601, cl/348560, cl/371542 added JavaScriptCore testing (command line version of Safari's JS engine)
cl/426461, cl/501501, cl/501480 added / will add Safari browser testing to Dart CI

flutter-dashboard · 2026-03-18T13:26:51Z

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing. If you believe this PR qualifies for a test exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!). The test exemption team is a small volunteer group, so all reviewers should feel empowered to ask for tests, without delegating that responsibility entirely to the test exemption group.

gemini-code-assist

Code Review

This pull request updates the WebAssembly compatibility check by replacing the supportsWasmGC function with a more comprehensive supportsDart2Wasm function. This new function checks for both WasmGC and js-string builtin support, which is a requirement for modern dart2wasm output. The changes are consistently applied across the JavaScript logic, its corresponding TypeScript definitions, and the loader.

mdebbar

LGTM with some questions and suggestions for leaving breadcrumbs.

I'll defer to @eyebrowsoffire to give the final approval here.

mdebbar · 2026-03-18T16:47:45Z

I believe the .ci.yaml validation error can be resolved by rebasing.

mkustermann · 2026-03-19T09:25:16Z

I believe the .ci.yaml validation error can be resolved by rebasing.

👍 Done

mkustermann · 2026-03-20T11:55:36Z

@eyebrowsoffire Could you have a look if you have some time?

eyebrowsoffire

LGTM

mdebbar

@mkustermann is this still something we want to land?

mkustermann · 2026-05-18T19:22:31Z

@mkustermann is this still something we want to land?

Absolutely. Though there's some failures from flutter CI bots I'd need to dig into (in case someone from flutter knows already what's wrong, please let me know!).

Before doing so, I decided to first add proper Safari tests on Dart CI first (which is still in-progress, as things always turn out to be more complicated then they should be). So far we have been only testing on the JS commandline version of Safari and only the newest version of it (i.e. bleeding edge JavaScriptCore JS shell).

kevmoo · 2026-05-18T19:32:45Z

Would you add a link to the top (original) PR comment linking to the pending Dart CL? Just so we keep our relationships straight. 🙏

mkustermann · 2026-05-18T19:53:35Z

Would you add a link to the top (original) PR comment linking to the pending Dart CL? Just so we keep our relationships straight. 🙏

Done

kevmoo · 2026-05-27T21:46:59Z

I believe this is blocked by https://github.com/flutter/flutter/pull/182861 – we need latest Chrome in our framework tests!

flutter-dashboard · 2026-06-16T21:47:42Z

This pull request executed golden file tests, but it has not been updated in a while (20+ days). Test results from Gold expire after as many days, so this pull request will need to be updated with a fresh commit in order to get results from Gold.

For more guidance, visit Writing a golden file test for package:flutter.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

gaaclarke · 2026-07-27T18:14:49Z

hey @mkustermann are you still interested in this PR? Just a heads up that it failed on CI, it sounds like you guys were waiting for some sort of chrome update that maybe is available now.

This changes flutter web's detection of wasm compatibility to be the expression emitted by dart2wasm. The expression was generated by passing `--require-js-string-builtin` to the dart2wasm compiler. Newer versions of browsers all support the `js-string` builtins nowadays. Apps running on browsers without `js-string` builtin support will not run in a performant way, so we'd not want to run the wasm version on them anyway. This is a preparation for dart2wasm to require the `js-string` builtin by default (i.e. we'll remove the `--require-js-string-builtin` flag and also remove the polyfill for `js-string` builtins).

mkustermann · 2026-07-28T08:09:53Z

are you still interested in this PR? Just a heads up that it failed on CI, it sounds like you guys were waiting for some sort of chrome update that maybe is available now.

Thanks for the ping, @gaaclarke ! Yes, I'd like the PR to land, 🤞 that the Chrome update will make CI pass now

Rebased now.

mkustermann · 2026-07-28T10:46:51Z

@eyebrowsoffire It seems it needs another LGTM

mkustermann · 2026-08-05T13:28:47Z

@mdebbar Could you give another lgtm?

For Dart String operations to be performant we require `js-string` builtins - the polyfill is not going to be ver fast. The newest versions of all major browsers support this now. This removes the polyfill for `js-string` builtins and makes the `<app>.support.js` code test for presence of `js-string` builtins. The PR in [0] changes flutter engine wasm support detection code to check for `js-string` builtins. [0] flutter/flutter#183835 Change-Id: Id20996674e2f6ce421f679747f7c2694260aedbe Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/488840 Reviewed-by: Srujan Gaddam <srujzs@google.com> Commit-Queue: Martin Kustermann <kustermann@google.com>

github-actions Bot added engine flutter/engine related. See also e: labels. platform-web Web applications specifically labels Mar 18, 2026

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mkustermann requested review from eyebrowsoffire and kevmoo March 18, 2026 14:11

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mkustermann force-pushed the dart2wasm-support-expr branch from 2ecab18 to ff74902 Compare March 19, 2026 09:22

mkustermann force-pushed the dart2wasm-support-expr branch from ff74902 to bbfba85 Compare March 20, 2026 11:56

eyebrowsoffire previously approved these changes Apr 28, 2026

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mkustermann force-pushed the dart2wasm-support-expr branch from bbfba85 to 849adea Compare April 28, 2026 18:56

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[web] Enable WebAssembly on latest Safari (Safari 26+) #186600

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mkustermann dismissed stale reviews from mdebbar and eyebrowsoffire via 512fa04 July 28, 2026 08:07

mkustermann force-pushed the dart2wasm-support-expr branch from 849adea to 512fa04 Compare July 28, 2026 08:07

github-actions Bot added the team-web Owned by Web platform team label Jul 28, 2026

Merge branch 'master' into dart2wasm-support-expr

867760c

mkustermann enabled auto-merge July 29, 2026 07:50

mkustermann mentioned this pull request Aug 5, 2026

[dart2wasm] Use new exception instructions dart-lang/sdk#54394

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Merge branch 'master' into dart2wasm-support-expr

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mdebbar approved these changes Aug 5, 2026

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mkustermann added this pull request to the merge queue Aug 5, 2026

Merged via the queue into flutter:master with commit 4c5bf76 Aug 5, 2026
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mkustermann deleted the dart2wasm-support-expr branch August 5, 2026 22:23

gmackall mentioned this pull request Aug 18, 2026

Update chrome version in ci.yaml to (probably) unblock tree flutter/packages#12498

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mkustermann commented Mar 18, 2026 •

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