Discussed during the last Impeller Weekly, this test harness will allow exercising dart:ui APIs to render content using either the Impeller or legacy rendering backend using SwiftShader (likely OpenGL ES or Vulkan).

Requirements

• Allows Dart fixtures to be written that use dart:ui APIs.
  • An existing harness that looks the closest to this today is embedder_unittests. Though, rendering is not its primary focus and the few pixel tests it has are mac guyver'd crudely.
• Allows exercising the Impeller rendering backend using SwiftShader.
  • Likely just Vulkan to begin with.
• The harness talks to the Flutter Engines instance of Skia Gold directly.
  • Should be doable since we already have a Dart isolate running during test execution.

This should also be the sole test harness in the engine that talks to Skia Gold and the one way of doing pixel tests. One this harness exists, existing tests can be migrated to use it (#135695).