refactor(upgrade): avoid mutable exports. (angular#34232)

Previously, create_angular_testing_module would export a mutable `let`
binding. The binding is already exporting using an accessor function
though, so the export on the let variable seems like an accidental
oversight.

This is functionally equivalent, but makes it easier for module
optimizers such as Closure Compiler to track down side effects and prune
modules.

PR Close angular#34232

packages/upgrade/static/testing/src/create_angular_testing_module.ts

@@ -12,7 +12,7 @@ import * as angular from '../../../src/common/src/angular1';
 import {$INJECTOR, INJECTOR_KEY, UPGRADE_APP_TYPE_KEY} from '../../../src/common/src/constants';
 import {UpgradeAppType} from '../../../src/common/src/util';
 
-export let $injector: angular.IInjectorService|null = null;
+let $injector: angular.IInjectorService|null = null;
 let injector: Injector;
 
 export function $injectorFactory() {