I guess that rakuco may be interested in this issue, too.  So I assign this to rakuco.
But please feel free to unassign yourself if this is not the case.

I'm definitely interested in this, I'm just not sure yet how to best implement this.

At the moment, we have functions like Header.entries() return an Iterator that's defined in core/Iterator.{h,idl}, which uses the standard IDL machinery to install a "next" method in its prototype and whose prototype inherits from the default Object prototype object. We end up with the following hierarchy:

    %ObjectPrototype%
       ^
       |
       |
    Iterator's prototype (with next(), @@iterator)
    Configured by the IDL machinery
       ^
       |
       |
    Iterator object (empty)
    in core/Iterator.{h,idl}

whereas it should actually look like this:

    %ObjectPrototype%
       ^
       |
       |
    %IteratorPrototype% (has @@iterator)
    comes from V8
       ^
       |
       |
    Iterator's prototype (with next() only)
       ^
       |
       |
    Iterator object (empty)
    in core/Iterator.{h,idl}

exposing %IteratorPrototype% from V8 is not very difficult, but changing Iterator, Iterator's prototype and making the latter inherit from IteratorPrototype is more challenging.

(extra bonus points if this allows us to get rid of the "Iterable" extended IDL attribute)

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/5ec1cddcdd0c599b00ae8f4fbb54987f92867b12

commit 5ec1cddcdd0c599b00ae8f4fbb54987f92867b12
Author: raphael.kubo.da.costa <raphael.kubo.da.costa@intel.com>
Date: Fri Apr 07 08:33:57 2017

Expose %IteratorPrototype% as an intrinsic in the public API.

The WebIDL spec expects iterator objects from interfaces that declare pair
iterators to ultimately inherit from %IteratorPrototype%. Expose the
intrinsic object in the public API so we can use it in Blink's bindings
code.

BUG= chromium:689576 
R=caitp@igalia.com,jkummerow@chromium.org,jochen@chromium.org

Review-Url: https://codereview.chromium.org/2784543004
Cr-Commit-Position: refs/heads/master@{#44472}

[modify] https://crrev.com/5ec1cddcdd0c599b00ae8f4fbb54987f92867b12/include/v8.h
[modify] https://crrev.com/5ec1cddcdd0c599b00ae8f4fbb54987f92867b12/test/cctest/test-api.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6579200aa9189bce1cd61c05fb6eb4493f2302d8

commit 6579200aa9189bce1cd61c05fb6eb4493f2302d8
Author: raphael.kubo.da.costa <raphael.kubo.da.costa@intel.com>
Date: Mon Apr 10 11:54:08 2017

Make pair iterators inherit from %IteratorPrototype%.

Adjust the prototype chain for iterator objects returned by interfaces that
have pair iterators.

The WebIDL spec says when an interface has pair iterators the iterators it
returns must be instances of the "default iterator object" whose
[[Prototype]] points to an "iterator prototype object" whose [[Prototype]],
on its turn, points to %IteratorPrototype%. next() must be implemented in
the "iterator prototype object", while %IteratorPrototype% provides
@@iterator.

In practice, this means the following should return true but was returning
false:

    Object.getPrototypeOf(Object.getPrototypeOf(new Headers().entries())) === Object.getPrototypeOf(Object.getPrototypeOf([][Symbol.iterator]()))

Previously, we just had the Iterator interface with both next() and
@@iterator added to its prototype object, which just inherited from
Object.prototype. We now adhere to the spec by making Iterator's prototype
object inherit by %IteratorPrototype% (which then inherits from
Object.prototype) instead. It also allows us to remove support for the
non-standard "Iterable" extended attribute we had in Blink.

The way we do it is not very pretty though: when the code for the Iterator
interface is generated, we create another function template with no
prototype and set the "prototype" property of its function object. When
|interfaceTemplate| is instantiated, its prototype.__proto__ will point to
the value of the "prototype" property we set, which is exactly what we want.

BUG= 689576 
R=bashi@chromium.org,haraken@chromium.org,yukishiino@chromium.org

Review-Url: https://codereview.chromium.org/2777183004
Cr-Commit-Position: refs/heads/master@{#463224}

[add] https://crrev.com/6579200aa9189bce1cd61c05fb6eb4493f2302d8/third_party/WebKit/LayoutTests/bindings/pair-iterators.html
[modify] https://crrev.com/6579200aa9189bce1cd61c05fb6eb4493f2302d8/third_party/WebKit/LayoutTests/external/wpt/fetch/api/headers/headers-basic-expected.txt
[modify] https://crrev.com/6579200aa9189bce1cd61c05fb6eb4493f2302d8/third_party/WebKit/Source/bindings/IDLExtendedAttributes.md
[modify] https://crrev.com/6579200aa9189bce1cd61c05fb6eb4493f2302d8/third_party/WebKit/Source/bindings/IDLExtendedAttributes.txt
[modify] https://crrev.com/6579200aa9189bce1cd61c05fb6eb4493f2302d8/third_party/WebKit/Source/bindings/scripts/v8_interface.py
[modify] https://crrev.com/6579200aa9189bce1cd61c05fb6eb4493f2302d8/third_party/WebKit/Source/bindings/templates/interface_base.cpp.tmpl
[modify] https://crrev.com/6579200aa9189bce1cd61c05fb6eb4493f2302d8/third_party/WebKit/Source/bindings/tests/idls/core/TestInterface.idl
[modify] https://crrev.com/6579200aa9189bce1cd61c05fb6eb4493f2302d8/third_party/WebKit/Source/bindings/tests/results/core/V8TestInterface.cpp
[modify] https://crrev.com/6579200aa9189bce1cd61c05fb6eb4493f2302d8/third_party/WebKit/Source/bindings/tests/results/core/V8TestInterface.h
[modify] https://crrev.com/6579200aa9189bce1cd61c05fb6eb4493f2302d8/third_party/WebKit/Source/core/dom/Iterator.idl