Type checking is not working properly with promises.

[joeuy]

TypeScript Version: 2.2.1 / nightly (2.2.0-dev.201xxxxx)

Code

var p1: Promise<{ a: string }>;
var p2: Promise<{ a: string, b: string }>;
p2 = p1;

link to playground

Expected behavior:
Type error

Actual behavior:
No error

[RyanCavanaugh]

See #15104