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Follows #8897 (iOS HTTP/2), now on main. ## What Android gets the same app-wide shared sessions as iOS, backed by Cronet (`cronet_http`) instead of `NSURLSession`: - One Cronet engine per traffic class (API, media), so requests to a host multiplex over one HTTP/2 connection and gzip is inflated natively, off the Dart isolate. - HTTP/2 on, HTTP cache off, QUIC off. QUIC would be a separate behavior change, and `cronet_http`'s changelog lists DNS failures with QUIC on cellular. - Streamed uploads (media and voice notes) stay on `dart:io` on Android: `CronetClient` reads a request body into memory before sending it, which would break upload progress and copy large attachments in full. - If no Cronet provider is available (no Google Play services), the engine fails to build and the app keeps `dart:io`, as today. - Transport error classification covers Cronet failures alongside `NSURLSession` ones, so invite and pairing errors keep their friendly messages. ## Measurements Android emulator (API 36, arm64), profile builds of this branch and main, signed in to the Feedback community. 8 cold starts per build, alternating builds in blocks of 4. Times are for relay `/query` requests started in the first 8 s. | | main (`dart:io`) | this branch (Cronet) | |---|---|---| | First query response | 6.99 s | 1.99 s | | Startup burst settled | 8.36 s | 2.78 s | | Burst after the first connection | 1.44 s | 0.77 s | | Query latency after the first connection, median / p90 | 165 / 1567 ms | 106 / 158 ms | **Unexplained stall:** most of main's startup gap is a ~5 s stall before its first response on every cold start. Its cause is unconfirmed; my guess is `dart:io` loading its CA store on the emulator, which HTTP/2 itself wouldn't fix, so the first two rows likely overstate the win on real devices. The rows after the first connection exclude the stall and are the fairer comparison. I also tried emulated EDGE throttling, but the emulator's throttle did not slow its traffic, so there are no slow-link numbers yet. ## Regressions None found. Devices without Play services keep `dart:io`. Cleartext `http://` requests keep the handle's own `dart:io` client, as on iOS. ## Testing - Full mobile suite passes (`flutter test`, 4784 tests). - Automated app run on the emulator: onboarding, Feedback nsec import, channel and DM list load, and 16 cold starts with request timing. Media loading wasn't separately exercised. - No visible UI change, so no screenshots. --------- Signed-off-by: Tom Brow <tomb@squareup.com> Co-authored-by: Tom Brow <tomb@squareup.com>