Convert to async refactoring can introduce name collision with parameter

[mjbvz]

TypeScript Version: typescript@3.2.0-dev.20181114

Repo
For the code.

async function foo<T>(x: T): Promise<T> {
    return x;
}

function bar<T>(x: T): Promise<T> {
    return foo(x).then(foo)
}

1. Return convert to async on bar

bug
This results in:

async function foo<T>(x: T): Promise<T> {
    return x;
}

async function bar<T>(x: T): Promise<T> {
    const x = await foo(x);
    return foo(x);
}

With an error because the new const x in bar collides with the x parameter