fix(upgrade): fallback to root ng2 injector when element is compiled outside the document (#8684)

Currently downgraded ng2 elements fail inside a ui-router view because they are unable
to require an ng2 Injector via the require attribute of the DDO, because ui-router compiles
its templates before they are inserted in a ui-view. This adds a "fallback" behavior if
a parent injector cannot be found to go to the root ng2 Injector.

modules/@angular/upgrade/test/upgrade_spec.ts

@@ -256,6 +256,37 @@ export function main() {
                         async.done();
                       })});
          }));
+
+
+      it('should fallback to the root ng2.injector when compiled outside the dom',
+        inject([AsyncTestCompleter], (async) => {
+          var adapter: UpgradeAdapter = new UpgradeAdapter();
+          var ng1Module = angular.module('ng1', []);
+
+          ng1Module.directive('ng1', ['$compile', ($compile) => {
+            return {
+              link: function($scope, $element, $attrs) {
+                var compiled = $compile("<ng2></ng2>");
+                var template = compiled($scope);
+                $element.append(template);
+              }
+            };
+          }]);
+
+          var Ng2 = Component({ selector: 'ng2', template: 'test' })
+            .Class({
+              constructor: function() { }
+            });
+          ng1Module.directive('ng2', adapter.downgradeNg2Component(Ng2));
+          var element = html('<ng1></ng1>');
+          adapter.bootstrap(element, ['ng1'])
+            .ready((ref) => {
+              expect(multiTrim(document.body.textContent))
+                .toEqual('test');
+              ref.dispose();
+              async.done();
+            });
+        }));
     });
 
     describe('upgrade ng1 component', () => {