Hi Dominic,

If you are busy I can help with this one. As you said
option 1 from https://github.com/w3c/webcomponents/issues/569 looks good to try.

Patch up at https://codereview.chromium.org/2443543002

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a21493ce8e12e552cd51c85523d451357bd427e1

commit a21493ce8e12e552cd51c85523d451357bd427e1
Author: dominicc <dominicc@chromium.org>
Date: Tue Oct 25 09:44:59 2016

createElement should not transmit exceptions but report them.

BUG= 652157 , 658066 

Review-Url: https://codereview.chromium.org/2443543002
Cr-Commit-Position: refs/heads/master@{#427306}

[modify] https://crrev.com/a21493ce8e12e552cd51c85523d451357bd427e1/third_party/WebKit/LayoutTests/custom-elements/spec/construct.html
[modify] https://crrev.com/a21493ce8e12e552cd51c85523d451357bd427e1/third_party/WebKit/LayoutTests/custom-elements/spec/create-element-defined-synchronous.html
[modify] https://crrev.com/a21493ce8e12e552cd51c85523d451357bd427e1/third_party/WebKit/LayoutTests/custom-elements/spec/report-the-exception.html
[modify] https://crrev.com/a21493ce8e12e552cd51c85523d451357bd427e1/third_party/WebKit/LayoutTests/custom-elements/spec/resources/custom-elements-helpers.js
[delete] https://crrev.com/d3440cefcf916da441e0a3f417b6664ea5d0fd52/third_party/WebKit/LayoutTests/imported/wpt/custom-elements/Document-createElement-expected.txt
[modify] https://crrev.com/a21493ce8e12e552cd51c85523d451357bd427e1/third_party/WebKit/Source/bindings/core/v8/ScriptCustomElementDefinition.cpp
[modify] https://crrev.com/a21493ce8e12e552cd51c85523d451357bd427e1/third_party/WebKit/Source/bindings/core/v8/ScriptCustomElementDefinition.h
[modify] https://crrev.com/a21493ce8e12e552cd51c85523d451357bd427e1/third_party/WebKit/Source/core/dom/Document.cpp
[modify] https://crrev.com/a21493ce8e12e552cd51c85523d451357bd427e1/third_party/WebKit/Source/core/dom/custom/CustomElement.cpp
[modify] https://crrev.com/a21493ce8e12e552cd51c85523d451357bd427e1/third_party/WebKit/Source/core/dom/custom/CustomElement.h
[modify] https://crrev.com/a21493ce8e12e552cd51c85523d451357bd427e1/third_party/WebKit/Source/core/dom/custom/CustomElementDefinition.h
[modify] https://crrev.com/a21493ce8e12e552cd51c85523d451357bd427e1/third_party/WebKit/Source/core/dom/custom/CustomElementTestHelpers.h