[polymer] add typings for light dom functions

documented here: https://polymer-library.polymer-project.org/1.0/docs/devguide/local-dom#light-dom-children

types/polymer/index.d.ts

@@ -218,6 +218,10 @@ declare global {
       detached?(): void;
 
       attributeChanged?(name: string, oldValue: any, newValue: any): void;
+        
+      getEffectiveChildren?(): Node[];
+
+      getEffectiveChildNodes?(): Node[];
     }
 
     // This is the type of a Polymer element after it has gone through the
@@ -277,6 +281,10 @@ declare global {
       observeNodes(callback: (info: ObservedNodeInfo) => void): {};
 
       unobserveNodes(observer: {}): void;
+        
+      getEffectiveChildren(): Node[];
+
+      getEffectiveChildNodes(): Node[];
 
       childNodes:Node[];