29593 [Web] Reland: Fix BMP encoder (cla: yes, platform-web)

29597 Add a samplerUniforms field for FragmentProgram (affects: engine, cla: yes, waiting for tree to go green, platform-web)

29622 [web] Fix warning about the non-nullable flag (cla: yes, waiting for tree to go green, platform-web, needs tests)

29638 [Embedder] Send key data through message channel (platform-ios, cla: yes, platform-web, platform-fuchsia, embedder)

29666 [web] move all build artifacts under web_ui/build (cla: yes, platform-web)

29680 Use a prebuilt Dart SDK during Fuchsia builds (cla: yes, platform-web)