
PASS Element.attachShadow must create an instance of ShadowRoot for autonomous custom elements 
PASS Element.attachShadow must create an instance of ShadowRoot for undefined autonomous custom elements 
FAIL Element.attachShadow for an autonomous custom element with disabledFeatures=["shadow"] should throw a NotSupportedError assert_throws: Definition, not a host function "() => {
    document.createElement('shadow-disabled-element').attachShadow({mode: 'closed'});
  }" did not throw
FAIL Element.attachShadow for a customized built-in element with disabledFeatures=["shadow"] should throw a NotSupportedError assert_throws: Definition, not a host function "() => {
    h2.attachShadow({mode: 'closed'});
  }" did not throw
PASS Element.attachShadow for a custom element with disabledFeatures=["SHADOW"] should not throw a NotSupportedError 
PASS Element.attachShadow must throw a NotSupportedError for customized built-in elements