Fix for key: T, value in loop to define value using typed key

[edemaine]

Helps with #1562

Before:

for x: T, y in z
  console.log x, y
---
for (const key in z) {const x: T = key;const y = z[key];
  console.log(x, y)
}

After:

for x: T, y in z
  console.log x, y
---
for (const key in z) {const x: T = key;const y = z[x];
  console.log(x, y)
}

This allows us to manually fix the type of the key to keyof typeof z, for example, which helps get the right type for y.