Update polymer.d.ts: fix es6 class syntax

polymer/polymer-tests.ts

@@ -63,7 +63,7 @@ var el2 = document.createElement('my-element');
 class MyElement2 {
   is: string;
 
-  registered() {
+  beforeRegister() {
     this.is = "my-element2";
   }
 }
@@ -74,7 +74,7 @@ Polymer(MyElement2);
 class MyElement3 implements polymer.Base {
   is: string;
 
-  registered() {
+  beforeRegister() {
     this.is = "my-element3";
   }
 }

polymer/polymer.d.ts

@@ -201,6 +201,8 @@ declare module polymer {
 
     observers?: string[];
 
+    beforeRegister?(): void;
+
     registered?(): void;
 
     created?(): void;