Address review comments

- Throw on repeat call to `attachInternals()`.
- Add warning for `checkValidity()` calls.
- Retype and rename `ariaMixinEnum` to `ariaMixinAttributes`.
- Loop through ariaAttrMap instead of internals instance.

packages/labs/ssr-dom-shim/src/index.ts

@@ -6,7 +6,7 @@
 import {ElementInternalsShim} from './lib/element-internals.js';
 
 export {
-  ariaMixinEnum,
+  ariaMixinAttributes,
   ElementInternals,
   HYDRATE_INTERNALS_ATTR_PREFIX,
 } from './lib/element-internals.js';
@@ -73,6 +73,12 @@ const ElementShim = class Element {
     return shadowRoot;
   }
   attachInternals(): ElementInternals {
+    if (this.__internals !== null) {
+      throw new Error(
+        `Failed to execute 'attachInternals' on 'HTMLElement': ` +
+          `ElementInternals for the specified element was already attached.`
+      );
+    }
     const internals = new ElementInternalsShim(this as unknown as HTMLElement);
     this.__internals = internals;
     return internals as ElementInternals;

packages/labs/ssr-dom-shim/src/lib/element-internals.ts

@@ -16,7 +16,14 @@ declare global {
   }
 }
 
-export const ariaMixinEnum: Record<keyof ARIAMixin, string> = {
+type ARIAAttributeMap = {
+  [K in keyof ARIAMixin]: string;
+};
+
+/**
+ * Map of ARIAMixin properties to attributes
+ */
+export const ariaMixinAttributes: ARIAAttributeMap = {
   ariaAtomic: 'aria-atomic',
   ariaAutoComplete: 'aria-autocomplete',
   ariaBraileLabel: 'aria-brailelabel',
@@ -119,6 +126,12 @@ export const ElementInternalsShim = class ElementInternals
     this.__host = _host;
   }
   checkValidity() {
+    // TODO(augustjk) Consider actually implementing logic.
+    // See https://github.com/lit/lit/issues/3740
+    console.warn(
+      '`ElementInternals.checkValidity()` was called on the server.' +
+        'This method always returns true.'
+    );
     return true;
   }
   form = null;

packages/labs/ssr/src/lib/lit-element-renderer.ts

@@ -8,7 +8,7 @@ import {ElementRenderer} from './element-renderer.js';
 import {LitElement, CSSResult, ReactiveElement} from 'lit';
 import {_$LE} from 'lit-element/private-ssr-support.js';
 import {
-  ariaMixinEnum,
+  ariaMixinAttributes,
   HYDRATE_INTERNALS_ATTR_PREFIX,
 } from '@lit-labs/ssr-dom-shim';
 import {renderValue} from './render-value.js';
@@ -43,13 +43,11 @@ export class LitElementRenderer extends ElementRenderer {
       this.element as object as {__internals: ElementInternals}
     ).__internals;
     if (internals) {
-      for (const [key, value] of Object.entries(internals)) {
-        const ariaAttribute = ariaMixinEnum[key as keyof ARIAMixin];
-        if (
-          ariaAttribute &&
-          value &&
-          !this.element.hasAttribute(ariaAttribute)
-        ) {
+      for (const [ariaProp, ariaAttribute] of Object.entries(
+        ariaMixinAttributes
+      )) {
+        const value = internals[ariaProp as keyof ARIAMixin];
+        if (value && !this.element.hasAttribute(ariaAttribute)) {
           this.element.setAttribute(ariaAttribute, value);
           this.element.setAttribute(
             `${HYDRATE_INTERNALS_ATTR_PREFIX}${ariaAttribute}`,