Add missing member codefix changes lib files

[andrewbranch]

declare let p: Promise<string>;
async function f() {
  p.toLowerCase();
}

Available fixes are

1. Add await
2. Declare method 'toLowerCase'
3. Declare property 'toLowerCase'
4. Add index signature for property 'toLowerCase'

2–4 modify the Promise declaration in lib.es5.d.ts.

I recently suggested to @sandersn that a new codefix shouldn’t change any .d.ts files, since they’re likely to be downloaded or autogenerated. That may or may not be too aggressive a rule for all codefixes, but I think it may be worth asserting that no codefixes change lib files or node_modules.

[DanielRosenwasser]

I think it's nice to be given the opportunity to add members to .d.ts files in case they're not yet on DT. This

I think it may be worth asserting that no codefixes change lib files or node_modules.

Seems reasonable though.