'for..in' for null/undefined expression should not be an error in --strictNullChecks

[rbuckton]

// @strictNullChecks: true
function f(x: { [key: string]: number; } | null | undefined) {
 for (const key in x) { // 1
 console.log(x[key]); // 2
 }
}

Expected behavior:

The for..in statement in ECMAScript does not throw if the expression is undefined or null, but rather the loop body is not evaluated. As such, the correct behavior under --strictNullChecks should be:

• The for..in statement (1) should be legal, even if x is undefined or null.
• Inside the for..in statement, the x (2) should no longer be considered undefined or null.

Actual behavior:

• The for..in statement (1) reports the error: Object is possibly 'null' or 'undefined'.
• Inside the for..in statement, the x (2) reports the error: Object is possibly 'null' or 'undefined'.