This PR fixes#2275
There are keywords that are allowed as identifiers.
https://tc39.es/ecma262/#sec-keywords-and-reserved-words
> Those that are always allowed as identifiers, but also appear as keywords within certain syntactic productions, at places where Identifier is not allowed: as, async, from, get, meta, of, set, and target.
This PR adds test cases for them, and fixes some cases such as
`class A { set(a, b) { } }`
`function of() { }`
`let obj = {async: true}`
`async()`
Choongwoo Han
2072f51faf