Emit Iterable<T> for sequence type arguments for es6 lib target

[saschanaz]

Input:

interface Foo {
  void bar(sequence<short> numbers);
};

dom.generated.d.ts:

interface Foo {
  bar(numbers: number[]): void;
}

dom.es6.generated.d.ts:

interface Foo {
  bar(numbers: Iterable<number>): void;
}

This will fix #419 in more general way, although still we have no choice for dictionary types.

[saschanaz]

Fixed by #699. Yay me.