feat(ngUpgrade): add support for AoT compiled upgrade applications

This commit introduces a new API to the ngUpgrade module, which is compatible
with AoT compilation. Primarily, it removes the dependency on reflection
over the Angular 2 metadata by introducing an API where this information
is explicitly defined, in the source code, in a way that is not lost through
AoT compilation.

This commit is a collaboration between @mhevery (who provided the original
design of the API); @gkalpak & @petebacondarwin (who implemented the
API and migrated the specs from the original ngUpgrade tests) and @alexeagle
(who provided input and review).

This commit is an starting point, there is still work to be done:

* add more documentation
* validate the API via internal projects
* align the ngUpgrade compilation of A1 directives closer to the real A1
  compiler
* add more unit tests
* consider support for async `templateUrl` A1 upgraded components

Closes #12239

modules/@angular/upgrade/test/aot/downgrade_injectable_spec.ts

@@ -0,0 +1,24 @@
+/**
+ * @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
+ */
+
+import {INJECTOR_KEY} from '@angular/upgrade/src/aot/constants';
+import {downgradeInjectable} from '@angular/upgrade/src/aot/downgrade_injectable';
+
+export function main() {
+  describe('downgradeInjectable', () => {
+    it('should return an Angular 1 annotated factory for the token', () => {
+      const factory = downgradeInjectable('someToken');
+      expect(factory[0]).toEqual(INJECTOR_KEY);
+      expect(factory[1]).toEqual(jasmine.any(Function));
+      const injector = {get: jasmine.createSpy('get').and.returnValue('service value')};
+      const value = (factory as any)[1](injector);
+      expect(injector.get).toHaveBeenCalledWith('someToken');
+      expect(value).toEqual('service value');
+    });
+  });
+}