feat(web): migrate web bindings to dart:js_interop for WebAssembly (dart2wasm) support

[sunweiyang]

Problem

flutter build web --wasm builds that include this package compile successfully but break at runtime: the platform implementation is selected via

export 'iris_method_channel_internal_expect.dart'
    if (dart.library.io) 'iris_method_channel_internal_actual_io.dart'
    if (dart.library.js) 'iris_method_channel_internal_actual_web.dart';

and under dart2wasm both dart.library.io and dart.library.js are false (wasm exposes dart.library.js_interop instead), so the stub implementation is exported and createAgoraRtcEngine() in agora_rtc_engine throws UnimplementedError: Unimplemented createPlatformBindingsProvider before any API call runs. Because this is a silent wrong-import selection rather than a compile error, apps only discover it in production wasm builds.

Changes

• Conditional imports in iris_method_channel_internal.dart and utils.dart: dart.library.js → dart.library.js_interop, which is true under both dart2js and dart2wasm, so a single web implementation serves both compilers (including the JS fallback that --wasm builds ship for non-wasmGC browsers).
• lib/src/platform/web/bindings/iris_api_common_bindings_js.dart: migrated from legacy package:js @JS()/@anonymous classes to dart:js_interop extension types (legacy package:js/dart:js are unavailable under dart2wasm):
  • array-typed members are now JSArray — under dart2wasm Dart Lists are not JS arrays, so implicit passthrough no longer exists; callers convert with .toJS/.toDart.
  • callIrisApi is now typed JSPromise<CallIrisApiResult> (what IrisCore.callIrisApi actually returns) instead of int + a dartify() Promise cast, which does not work under dart2wasm.
  • IrisCEventHandler is now a JSFunction typedef; iris_method_channel_internal_web.dart uses .toJS instead of allowInterop.
• pubspec.yaml: SDK floor raised to >=3.3.0 (required for dart:js_interop extension types). The language-version bump also requires the ffigen-generated ffi.Struct subclasses to be final (Dart 3 class modifiers) — io-only files, no behavior change.

Breaking-change note

The exported web bindings surface (iris_method_channel_bindings_web.dart) changes shape (JS types instead of Dart lists/functions). Its consumer in agora_rtc_engine needs the matching one-file update to its web binding delegate — I will submit that companion PR to Agora-Flutter-SDK referencing this one. test/platform/fake still uses legacy interop and is untouched in this PR; happy to migrate it as well if you'd like.

Testing

• dart analyze lib clean (errors/warnings).
• End-to-end smoke test in a production Flutter app (agora_rtc_engine 6.6.3 + iris-web-rtc 0.8.1, headless Chrome with fake media devices): under both the wasm and dart2js runtimes, createAgoraRtcEngine → initialize → registerEventHandler → enableAudio → joinChannel all succeed, and the JS→Dart event path delivers typed events (including the buffer marshalling in both directions). dart2js behavior is unchanged.