Extending native types works for internal modules, but doesn't for external

[AndreiPashkin]

This one works as expected:

interface Array<T> {
    foo(): number;
}

Array.prototype.foo = function(): number {
    return 42;
}

var arr: string[] = ['a', 'b', 'c'];
var x = arr.foo();

But tsc throws an error for this one:

interface Array<T> {
    foo(): number;
}

Array.prototype['foo'] = function(): number {
    return 42;
}

var arr: string[] = ['a', 'b', 'c'];
var x = arr.foo();
export = x;

The error is: error TS2339: Property 'foo' does not exist on type 'string[]'.

[danquirk]

See #4166