[polymer] Expose polymer.PolymerElement type in Polymer 1.x typings. (#31407)

* [polymer] Expose polymer.PolymerElement type in Polymer 1.x typings.

This is a type that represents a Polymer element after its class has been constructed by the Polymer() function.

Previously people would construct such a type by doing:

```typescript
interface MyElement extends HTMLElement, polymer.Base {}
```

But after TypeScript 3.0, the type of `HTMLElement#toggleAttribute` is different than the type of `polymer.Base#toggleAttribute`, which makes it an error to declare the MyElement type as above.

Unfortunately, we can't change the type of toggleElement on polymer.Base, so instead we introduce a new type polymer.PolymerElement that already extends `HTMLElement`.

So after this change, a Polymer 1.0 user can write their type as:

```typescript
interface MyElement extends polymer.PolymerElement {}
```

* Fix a couple of small issues.

types/polymer/index.d.ts

@@ -20,19 +20,17 @@ declare global {
       observer?: string;
     }
 
-    interface Base {
-      /** Need to allow all properties for callback methods. */
-      [prop: string]: any;
-
+    interface CommonBase {
       /* polymer-micro */
 
       // Attributes
 
-      hostAttributes?: {[name:string]:any};
+      hostAttributes?: {[name: string]: any};
 
       reflectPropertiesToAttribute?(name: string): void;
 
-      serializeValueToAttribute?(value: any, attribute: string, node?: Element): void;
+      serializeValueToAttribute?
+          (value: any, attribute: string, node?: Element): void;
 
       deserialize?(value: string, type: NumberConstructor): number;
       deserialize?(value: string, type: BooleanConstructor): boolean;
@@ -120,7 +118,8 @@ declare global {
 
       notifyPath?(path: string, value: any, fromAbove: any): void;
 
-      set?<Value>(path: string|(string|number)[], value: Value, root?: Object): void;
+      set?<Value>(path: string|(string|number)[], value: Value, root?: Object):
+          void;
 
       get?(path: string|(string|number)[], root?: Object): any;
 
@@ -139,7 +138,8 @@ declare global {
 
       unshift?(path: string, ...item: any[]): number;
 
-      notifySplices?(path: string, splices: ReadonlyArray<polymer.PolymerSplice>): void;
+      notifySplices?
+          (path: string, splices: ReadonlyArray<polymer.PolymerSplice>): void;
 
       // ResolveUrl
 
@@ -155,8 +155,6 @@ declare global {
 
       toggleClass?(name: string, bool?: boolean, node?: HTMLElement): void;
 
-      toggleAttribute?(name: string, bool?: boolean, node?: HTMLElement): void;
-
       classFollows?(name: string, toElement: HTMLElement, fromElement: HTMLElement): void;
 
       attributeFollows?(name: string, toElement: HTMLElement, fromElement: HTMLElement): void;
@@ -220,7 +218,19 @@ declare global {
       detached?(): void;
 
       attributeChanged?(name: string, oldValue: any, newValue: any): void;
+    }
 
+    // This is the type of a Polymer element after it has gone through the
+    // Polymer() function.
+    interface PolymerElement extends CommonBase, HTMLElement {}
+
+    interface Base extends CommonBase {
+      /** Need to allow all properties for callback methods. */
+      [prop: string]: any;
+
+      // Has to live on Base because it is incompatible with
+      // HTMLElement#toggleAttribute
+      toggleAttribute?(name: string, bool?: boolean, node?: HTMLElement): void;
     }
 
     interface DomApiStatic {

types/polymer/polymer-tests.ts

@@ -51,14 +51,14 @@ Polymer({
   }
 });
 
-var MyElement = Polymer.Class(<polymer.Base>{
+var MyElement = Polymer.Class({
   is: 'my-element',
 
   created: function () {
     this.textContent = 'My element!';
   }
 
-});
+} as polymer.Base);
 
 document.registerElement('my-element', MyElement);