fix(): avoid const / let

Author: manucorporat

src/client/polyfills/es5-html-element.js

@@ -7,7 +7,7 @@
   ) {
     return;
   }
-  const BuiltInHTMLElement = HTMLElement;
+  var BuiltInHTMLElement = HTMLElement;
   window.HTMLElement = /** @this {!Object} */ function HTMLElement() {
     return Reflect.construct(
         BuiltInHTMLElement, [], /** @type {!Function} */ (this.constructor));

src/client/polyfills/system.js

@@ -47,6 +47,7 @@ async function writeSystemLoader(config: d.Config, compilerCtx: d.CompilerCtx, l
 async function getSystemLoader(config: d.Config, corePath: string, includePolyfills: boolean) {
   const polyfills = includePolyfills ? await getAppBrowserCorePolyfills(config) : '';
   return `
+'use strict';
 ${polyfills}
 
 var doc = document;