[Fix] raise an error when a method name is in string form

[shijistar]

I got a compiling error when a component method name is in string form, but not an identifier. The sample code is,

class MyComponent {
    "special-method-name"() {
        // ...
    }
}

Normally method names should be valid identifiers, but for some reason, I need one of my methods in a special form. However any strings should be valid method name and both babel and Chrome run without any problem.

The difference between Identifier and String method is that the node.key.type is Identifier/Literal. So I made a special process here.

[ljharb]

Duplicate of #2514. it seems.

[shijistar]

Duplicate of #2514. it seems.

Yes, it's duplicated, and this PR should be ignored.