Deny ElementInternals.shadowRoot for pre-created shadow roots

Per the spec issue [1], this change disallows the use of
ElementInternals.shadowRoot for shadow roots created prior
to the custom element constructor being run. This protects
potentially-closed shadow roots, created outside of the
custom element, from being revealed to the custom element
itself. (Use case unclear, but this was the request.)

[1] WICG/webcomponents#871

Bug: 1042130
Change-Id: I25192256e8b1334d09ea587f29d64f35d4f5f949

shadow-dom/declarative/element-internals-shadowroot.tentative.html

@@ -108,6 +108,23 @@
   assert_throws_dom('NotSupportedError', () => element.attachInternals(), 'attachInternals forbidden by disabledFeatures, post-upgrade');
 }, 'ElementInternals disabled by disabledFeatures');
 
-
-
+test(() => {
+  let constructed = false;
+  const element = document.createElement('x-6');
+  const sr = element.attachShadow({mode: 'closed'});
+  assert_true(sr instanceof ShadowRoot);
+  customElements.define('x-6', class extends HTMLElement {
+    constructor() {
+      super();
+      assert_throws_dom('NotSupportedError', () => this.attachShadow({mode:'open'}), 'attachShadow already called');
+      const elementInternals = this.attachInternals();
+      assert_equals(elementInternals.shadowRoot, null, 'ElementInternals.shadowRoot should not be available for pre-attached shadow');
+      constructed = true;
+    }
+  });
+  assert_false(constructed);
+  customElements.upgrade(element);
+  assert_true(constructed,'Failed to construct - test failed');
+  assert_equals(element.shadowRoot, null, 'shadow root is closed');
+}, 'ElementInternals.shadowRoot doesn\'t reveal pre-attached closed shadowRoot');
 </script>