Call signature is not recognized as 'extra' in object literals

[anvish]

TypeScript Version: 2.1.6

Code

interface A {
    a?: string
}
const a1: A = { b: 2 } // correct: error "Object literal may only specify known properties"
const a2: A = () => { } // incorrect: no error

Expected behavior:
Call signature is treated like an excessive property, so both lines emit an error.
Actual behavior:
Only properties are checked, so only first line emits an error.

See #3755

[mhegazy]

Should be working in latest.