Re implment the CustomElementRegistry interface

Author: startracex

packages/custom-elements/ts_src/CustomElementRegistry.ts

@@ -278,6 +278,11 @@ export default class CustomElementRegistry {
     return undefined;
   }
 
+  getName(constructor: ElementConstructor): string | null {
+    const definition = this._constructorToDefinition.get(constructor);
+    return definition ? definition.localName : null;
+  }
+
   whenDefined(localName: string): Promise<void> {
     if (!Utilities.isValidCustomElementName(localName)) {
       return Promise.reject(
@@ -363,4 +368,6 @@ CustomElementRegistry.prototype['polyfillDefineLazy'] =
   CustomElementRegistry.prototype.polyfillDefineLazy;
 CustomElementRegistry.prototype['polyfillWrapFlushCallback'] =
   CustomElementRegistry.prototype.polyfillWrapFlushCallback;
+CustomElementRegistry.prototype['getName'] =
+  CustomElementRegistry.prototype.getName;
 /* eslint-enable no-self-assign */

packages/tests/custom-elements/html/registry.test.html

@@ -209,6 +209,23 @@
             });
           });
 
+          suite('getName', function () {
+            test('returns the local name for a defined custom element constructor', function () {
+              class XGetName extends HTMLElement {}
+              customElements.define('x-get-name', XGetName);
+
+              var name = customElements.getName(XGetName);
+              assert.equal(name, 'x-get-name');
+            });
+
+            test('returns null for an undefined custom element constructor', function () {
+              class XUndefinedElement extends HTMLElement {}
+
+              var name = customElements.getName(XUndefinedElement);
+              assert.isNull(name);
+            });
+          });
+
           suite('whenDefined', function () {
             test('resolves when a tag is defined', function () {
               var promise = customElements