fix: create proper externs so that closure does not clobber e.g. ng for internal variables (#19423)

packages/core/src/core.externs.js

@@ -0,0 +1,18 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ *
+ * @externs
+ */
+
+/**
+ * @suppress {duplicate}
+ */
+var ng;
+/**
+ * @suppress {duplicate}
+ */
+var wtf;

packages/core/src/util.ts

@@ -21,6 +21,11 @@ const __self = typeof self !== 'undefined' && typeof WorkerGlobalScope !== 'unde
     self instanceof WorkerGlobalScope && self;
 const __global = typeof global !== 'undefined' && global;
 const _global: {[name: string]: any} = __window || __global || __self;
+/**
+ * Attention: whenever providing a new value, be sure to add an
+ * entry into the corresponding `....externs.js` file,
+ * so that closure won't use that global for its purposes.
+ */
 export {_global as global};
 
 // When Symbol.iterator doesn't exist, retrieves the key used in es6-shim