28139 fuchsia: Improve FakeSession & add tests (affects: tests, cla: yes, waiting for tree to go green, platform-fuchsia, tech-debt)

28144 fuchsia: Add FuchsiaExternalViewEmbedder tests (affects: tests, cla: yes, platform-fuchsia, tech-debt)

28202 Revert “Add SPIR-V FragmentShader API to painting.dart” (cla: yes, platform-web, platform-fuchsia)

28207 Revert “Adds semantics tooltip support” (platform-ios, platform-android, cla: yes, platform-web, platform-fuchsia, embedder)

28208 [Re-land] Add SPIR-V FragmentShader API to painting.dart (cla: yes, platform-web, platform-fuchsia)