[lit-next] Minify private lit-element properties, and rename lit-html…

… private props to one underscore (lit#1334)

- Mangle all lit-element properties starting with `_`
- Change private lit-html property prefixes from `__` to `_` for consistency with lit-element
  - Includes exclusions for `_value`, `_setValue`, `_$litType$`, and `_$litDirective$`
- Pre-seed the terser name cache with random names for `_createElement`, `_endNode`, and `_startNode` since those will be needed by Steve in lit#1315. We can use this pattern to ensure that any private properties we need to access *across packages* will be consistent, including across versions.

Also some small typing improvements I noticed:
- Add a cast to `UpdatingElement` prototype in lit-element
- Replace an `any` cast in lit-html with a `NodePartInternal` cast

packages/lit-element/src/lib/updating-element.ts

@@ -396,9 +396,9 @@ export abstract class UpdatingElement extends HTMLElement {
     }
     this[finalized] = true;
     // finalize any superclasses
-    const superProto = Object.getPrototypeOf(this);
+    const superProto = Object.getPrototypeOf(this) as typeof UpdatingElement;
     superProto.finalize();
-    this._classProperties = new Map(superProto._classProperties);
+    this._classProperties = new Map(superProto._classProperties!);
     // initialize Map populated in observedAttributes
     this._attributeToPropertyMap = new Map();
     // make any properties

packages/lit-html/src/directives/repeat.ts

@@ -71,7 +71,7 @@ class RepeatDirective extends Directive {
     }
   }
 
-  __getValuesAndKeys<T>(
+  private _getValuesAndKeys<T>(
     items: Iterable<T>,
     keyFnOrTemplate: KeyFn<T> | ItemTemplate<T>,
     template?: ItemTemplate<T>
@@ -101,7 +101,7 @@ class RepeatDirective extends Directive {
     keyFnOrTemplate: KeyFn<T> | ItemTemplate<T>,
     template?: ItemTemplate<T>
   ) {
-    return this.__getValuesAndKeys(items, keyFnOrTemplate, template).values;
+    return this._getValuesAndKeys(items, keyFnOrTemplate, template).values;
   }
 
   update<T>(
@@ -115,7 +115,7 @@ class RepeatDirective extends Directive {
     // Old part & key lists are retrieved from the last update
     // TODO: deal with directive being swapped out?
     const oldParts = getPartValue(containerPart) as Array<NodePart | null>;
-    const {values: newValues, keys: newKeys} = this.__getValuesAndKeys(
+    const {values: newValues, keys: newKeys} = this._getValuesAndKeys(
       items,
       keyFnOrTemplate,
       template

packages/lit-html/src/directives/template-content.ts

@@ -21,7 +21,7 @@ import {
 } from '../lit-html.js';
 
 class TemplateContent extends Directive {
-  private __previousTemplate?: HTMLTemplateElement;
+  private _previousTemplate?: HTMLTemplateElement;
 
   constructor(part: PartInfo) {
     super();
@@ -31,10 +31,10 @@ class TemplateContent extends Directive {
   }
 
   render(template: HTMLTemplateElement) {
-    if (this.__previousTemplate === template) {
+    if (this._previousTemplate === template) {
       return noChange;
     }
-    this.__previousTemplate = template;
+    this._previousTemplate = template;
     return document.importNode(template.content, true);
   }
 }