fix(ng-view): cleanup should not destroy an already destroyed scope

Refer https://github.com/angular/angular.dart/issues/1182
and repro https://github.com/chirayuk/sample/tree/issue_1182_leaving_a_nested_ng_view

NgView's register cleanup handlers this way:

    _leaveSubscription = route.onLeave.listen((_) {
      _leaveSubscription.cancel();
      // …
      _cleanUp();
    });

When there are nested ng-views, upon a route change, the parent NgView
calls it's _cleanUp() first (which destroys it's child scope) and then
the child NgView attempts a cleanup.  However, it's child scope is
already detached due to the parent NgView cleaning up causing an
exception.

Stack trace is:

    'package:angular/core/scope.dart': Failed assertion: line 335 pos 12: 'isAttached' is not true.

    STACKTRACE:
    #0      Scope.destroy (package:angular/core/scope.dart:335:12)
    #1      NgView._cleanUp (package:angular/routing/ng_view.dart:130:24)
    #2      NgView._show.<anonymous closure> (package:angular/routing/ng_view.dart:106:15)
    #3      _rootRunUnary (dart:async/zone.dart:730)
    #4      _ZoneDelegate.runUnary (dart:async/zone.dart:462)
    #5      _onRunUnary.<anonymous closure> (package:angular/core/zone.dart:113:63)
    #6      VmTurnZone._onRunBase (package:angular/core/zone.dart:97:16)
    #7      _onRunUnary (package:angular/core/zone.dart:113:17)
    #8      _ZoneDelegate.runUnary (dart:async/zone.dart:462)
    #9      _CustomizedZone.runUnary (dart:async/zone.dart:667)
    #10     _BaseZone.runUnaryGuarded (dart:async/zone.dart:582)
    #11     _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:333)
    #12     _BufferingStreamSubscription._add (dart:async/stream_impl.dart:263)
    #13     _SyncBroadcastStreamController._sendData.<anonymous closure> (dart:async/broadcast_stream_controller.dart:344)
    #14     _BroadcastStreamController._forEachListener (dart:async/broadcast_stream_controller.dart:297)
    #15     _SyncBroadcastStreamController._sendData (dart:async/broadcast_stream_controller.dart:343)
    #16     _BroadcastStreamController.add (dart:async/broadcast_stream_controller.dart:227)
    #17     Router._leaveCurrentRouteHelper (package:route_hierarchical/client.dart:654:48)
    #18     Router._leaveCurrentRouteHelper (package:route_hierarchical/client.dart:656:47)
    #19     Router._leaveCurrentRoute (package:route_hierarchical/client.dart:645:41)
    #20     Router._leaveOldRoutes (package:route_hierarchical/client.dart:525:30)
    #21     Router._processNewRoute (package:route_hierarchical/client.dart:497:27)
    #22     Router._route.<anonymous closure> (package:route_hierarchical/client.dart:481:29)
    #23     _rootRunUnary (dart:async/zone.dart:730)
    #24     _ZoneDelegate.runUnary (dart:async/zone.dart:462)
    #25     _onRunUnary.<anonymous closure> (package:angular/core/zone.dart:113:63)
    #26     VmTurnZone._onRunBase (package:angular/core/zone.dart:97:16)
    #27     _onRunUnary (package:angular/core/zone.dart:113:17)
    #28     _ZoneDelegate.runUnary (dart:async/zone.dart:462)
    #29     _CustomizedZone.runUnary (dart:async/zone.dart:667)
    #30     _Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:488)
    #31     _Future._propagateToListeners (dart:async/future_impl.dart:571)
    #32     _Future._completeWithValue (dart:async/future_impl.dart:331)
    #33     _Future._asyncComplete.<anonymous closure> (dart:async/future_impl.dart:393)
    #34     _rootRun (dart:async/zone.dart:723)
    #35     _ZoneDelegate.run (dart:async/zone.dart:453)
    #36     _onScheduleMicrotask.<anonymous closure> (package:angular/core/zone.dart:117:43)
    #37     VmTurnZone._finishTurn (package:angular/core/zone.dart:143:34)
    #38     VmTurnZone._onRunBase (package:angular/core/zone.dart:104:43)
    #39     _onRunUnary (package:angular/core/zone.dart:113:17)
    #40     _ZoneDelegate.runUnary (dart:async/zone.dart:462)
    #41     _CustomizedZone.runUnary (dart:async/zone.dart:667)
    #42     _BaseZone.runUnaryGuarded (dart:async/zone.dart:582)
    #43     _BaseZone.bindUnaryCallback.<anonymous closure> (dart:async/zone.dart:608)

Closes #1182