Microsoft/TypeScript — pull request #944 by ahejlsberg (2014-10-23 00:06:01+00:00)
Title: In, for...in, and instanceof operators with union types

Adding support for union types with the `in` and `instanceof` operators and the `for...in` statement. We now check that an operand is of type any, an object type, a type parameter, or a union type composed exclusively of those kinds of types.