asm.js is standardized, in that its semantics are fully defined by ECMA-262, which is a multi-vendor, widely implemented standard.
I may be wrong, but I see Math.imul as part of the asm.js spec and I believe it's an extension on ECMA-252.5 (and related libraries, IIRC typed arrays are part of the webgl stuff, not ECMA-262)
Yeah, you're right. Both are part of the ES6 draft though: