==== tests/cases/compiler/genericArrayExtenstions.ts (2 errors) ====
    export declare class ObservableArray<T> implements Array<T> { // MS.Entertainment.ObservableArray
                         ~~~~~~~~~~~~~~~
!!! Cannot compile external modules unless the '--module' flag is provided.
                         ~~~~~~~~~~~~~~~
!!! Class 'ObservableArray<T>' incorrectly implements interface 'T[]':
!!!   Property 'length' is missing in type 'ObservableArray<T>'.
    concat<U extends T[]>(...items: U[]): T[];
    concat(...items: T[]): T[];
    }