[azakai]

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.

[johncolanduoni]

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.