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

Input:

```webidl
interface Foo {
  void bar(sequence<short> numbers);
};
```

dom.generated.d.ts:

```ts
interface Foo {
  bar(numbers: number[]): void;
}
```

dom.es6.generated.d.ts:

```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.