feat: allow for forward references in injection

It is possible for a class defined first to be referencing a class defined later,
and as a result at the time of the definition it is not possible to access the later's
class reference. This allows to refer to the later defined class through
a closure.Closes #1891

modules/angular2/test/di/forward_ref_spec.js

@@ -0,0 +1,23 @@
+import {
+  AsyncTestCompleter,
+  beforeEach,
+  ddescribe,
+  describe,
+  expect,
+  iit,
+  inject,
+  it,
+  xit,
+} from 'angular2/test_lib';
+import {forwardRef, resolveForwardRef} from 'angular2/di';
+import {Type} from 'angular2/src/facade/lang';
+
+export function main() {
+  describe("forwardRef", function () {
+    it('should wrap and unwrap the reference', () => {
+      var ref = forwardRef(() => String);
+      expect(ref instanceof Type).toBe(true);
+      expect(resolveForwardRef(ref)).toBe(String);
+    });
+  });
+}