Merge pull request #3930 from Wildhoney/add-is-attribute

Added 'is' attribute for custom elements

Author: sophiebits

src/renderers/dom/shared/HTMLDOMPropertyConfig.js

@@ -103,6 +103,7 @@ var HTMLDOMPropertyConfig = {
     httpEquiv: null,
     icon: null,
     id: MUST_USE_PROPERTY,
+    is: MUST_USE_ATTRIBUTE,
     keyParams: MUST_USE_ATTRIBUTE,
     keyType: MUST_USE_ATTRIBUTE,
     label: null,