Avoid using const keyword in the polyfill to support IE9-10

[soleo]

const will not work for IE11-

This is an issue introduced in v1.0.1 11 days ago, not an issue in v1.0.0

https://caniuse.com/#search=const

[soleo]

@sodatea Could you check this PR #2? This is causing some issues for vue-cli-service

[soleo]

related to vuejs/vue-cli#3760

[soleo]

Thanks! @sodatea

[soleo]

Are you planning to do a patch release soon?

[haoqunjiang]

Yeah, v1.0.2 published