I see very little point in Shumway.It will run most swf so slow it wont be really usable in a professional context.
Start from scratch.Use WebGL and emscriptem.Write a flash like IDE that uses C++ or a safe language that compiles with the LLVM toolchain.Make flash without flash,backed by webtechs instead writing an emulator or that's just cargo culting.
ActionScript compiled to JS is not the performance bottleneck. In many cases it is faster than ActionScript ever was simply because of the huge investment most browser vendors have put into their JS engines. Flash-like vector graphics is what's hard to emulate on the web today. If you only care about moving spites around, then Flash, Shumway and C++/OpenGL will be mostly GPU bound.
Flash got a lot of things right, both the SWF file format and the authoring tools are pretty good. There's not much sense in re-inventing it (we could do without ActionScript though).
Start from scratch.Use WebGL and emscriptem.Write a flash like IDE that uses C++ or a safe language that compiles with the LLVM toolchain.Make flash without flash,backed by webtechs instead writing an emulator or that's just cargo culting.