Declared type as initial type in control flow analysis

[ahejlsberg]

With this PR we consider the initial type of a variable to be a union of its declared type and undefined. This makes control flow based type analysis less aggressive in cases where the checker is unable to see assignments to a variable because they exclusively occur in nested functions (which aren't included in the analysis). For example:

let result: Foo | undefined;
scanForResult();
if (result) {
    // Do something with result
}

function scanForResult() {
    // Logic that possibly assigns to result
}

With the PR we consider the initial type of result to be Foo | undefined, which narrows to Foo in the type guard. Previously we considered the initial type to be undefined which would narrow to nothing and cause errors.

[mhegazy]

hmm.. non of the baselines changed.. did not expect that.

[mhegazy]

👍

[ahejlsberg]

@mhegazy Yeah, but that's partially because we don't have a lot of tests with --strictNullChecks. Still, it seems that it is rare to rely on the initial value in an uninitialized variable. Which, now that I write it, doesn't actually seem to surprising. 😄