asm.js is now validated and compiled to WebAssembly
If V8 encounters asm.js code it now tries to validate it. Valid asm.js code is then transpiled to WebAssembly. According to V8’s performance evaluations, this generally boosts throughput performance. Due to the added validation step, isolated regressions in startup performance might happen.
Read the full article: https://v8project.blogspot.jp/2017/08/v8-release-61.html