fix(core): always remove DOM listeners and stream subscriptions

This is needed to prevent memory leaks. The DOM
listeners don’t need to be removed for simple examples,
but a big internal app shows memory leaks because of them.

BREAKING CHANGE:
- `Renderer.listen` now has to return a function that
  removes the event listener.

modules/angular2/test/core/change_detection/change_detector_spec.ts

@@ -1267,7 +1267,7 @@ export function main() {
 
           val.changeDetector.dehydrate();
           expect(() => {val.changeDetector.detectChanges()})
-              .toThrowErrorWith("Attempt to detect changes on a dehydrated detector");
+              .toThrowErrorWith("Attempt to use a dehydrated detector");
           expect(val.dispatcher.log).toEqual(['propName=Bob']);
         });
       });

modules/angular2/test/core/linker/integration_spec.ts

@@ -877,13 +877,22 @@ function declareTests() {
                  var listener = tc.inject(DirectiveListeningEvent);
 
                  expect(listener.msg).toEqual('');
+                 var eventCount = 0;
 
                  ObservableWrapper.subscribe(emitter.event, (_) => {
-                   expect(listener.msg).toEqual('fired !');
-                   async.done();
+                   eventCount++;
+                   if (eventCount === 1) {
+                     expect(listener.msg).toEqual('fired !');
+                     fixture.destroy();
+                     emitter.fireEvent('fired again !');
+                   } else {
+                     expect(listener.msg).toEqual('fired !');
+                     async.done();
+                   }
                  });
 
                  emitter.fireEvent('fired !');
+
                });
          }));
 
@@ -961,6 +970,11 @@ function declareTests() {
                      .toEqual(
                          ['domEvent', 'body_domEvent', 'document_domEvent', 'window_domEvent']);
 
+                 fixture.destroy();
+                 listener.eventTypes = [];
+                 dispatchEvent(tc.nativeElement, 'domEvent');
+                 expect(listener.eventTypes).toEqual([]);
+
                  async.done();
                });
          }));