I suppose I should have asked "what does polyfill mean in this context?" Does that term really alias "run [it] on" or is there something more subtle about this term's meaning?
Web developers, bless their hearts, feel uncomfortable with the word "emulate", so they use "polyfill", or sometimes the more general "shim". To some degree, "polyfill" implies that the functionality being emulated/shimmed is functionality that is under some other circumstances is expected to be provided by the browser itself.
There's a high performance asm.js implementation of WASM (converting WASM to asm.js at runtime). ie if a browser didn't implement WASM, but had a JIT which executed asm.js at high speed, it could execute WASM at high speed
There isn't such a polyfill. There is currently no good tool to translate wasm into asm.js (or general JS) on the client. The best that exists is to run the wasm in a wasm interpreter, very slowly.
In theory a translator could be written into asm.js, but there would still be wasm code that won't run fast, such as 64-bit ints, unaligned loads and stores, bitcasts, and other operations.
Yes, but emscripten can compile to both (with or without using the new native LLVM WebAssembly backend). It's stretching the word polyfill, but emscripten users have pretty close to a turnkey solution for producing code that detects WebAssembly support and falls back to asm.js.