Fixes #555. Add CustomElementRegistry.prototype.get().

Author: treshugart

src/CustomElements/v1/CustomElements.js

@@ -150,6 +150,12 @@ var CustomElementDefinition;
       this._addNodes(doc.childNodes);
     }
 
+    // http://w3c.github.io/webcomponents/spec/custom/#dom-customelementsregistry-get
+    get(localName) {
+      const def = this._definitions.get(localName);
+      return def ? def.constructor : undefined;
+    }
+
     flush() {
       this._handleMutations(this._observer.takeRecords());
     }

tests/CustomElements/v1/js/customElements.js

@@ -313,6 +313,13 @@ suite('customElements', function() {
     done();
   });
 
+  test('customElements.get', function (done) {
+    class XBoo extensd HTMLElement {}
+    customElements.define('x-boo-get', XGetTest);
+    assert.equal(XBoo, customElements.get('x-boo-get'));
+    done();
+  });
+
   test('document.registerElement disconnectedCallbacks in prototype', function(done) {
     var ready, inserted, removed;
     class XBoo extends HTMLElement {