Fix type checking in matcher_types.js.
Fix type checking in matcher_types.js.

Adds explicit casts to clarify two cases where the type checker isn't quite sophisticated enough to infer the correct types.

Background: Closure Compiler is trying to land a change to improve inference of qualified names.  An upshot of this is that it will soon understand that "this.getDescription" is supposed to be a `function(): string` (rather than just unknown as it does currently), which causes a type error when it tries to return a `string|function(): string` (note that the compiler does not currently propagate the flow-sensitive context that `description` is definitely a string from outside the closure to inside).
google/gjstest,google/gjstest,google/gjstest,google/gjstest