Fix some observatory analysis issues.

runtime/observatory/lib/src/elements/native_memory_profiler.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library native_memory_profile;
 
 import 'dart:async';
 import 'dart:html';
 import 'package:observatory/models.dart' as M;
+import 'package:observatory/service.dart' as S;
 import 'package:observatory/src/elements/cpu_profile/virtual_tree.dart';
 import 'package:observatory/src/elements/helpers/nav_bar.dart';
 import 'package:observatory/src/elements/helpers/nav_menu.dart';
 import 'package:observatory/src/elements/helpers/rendering_scheduler.dart';
 import 'package:observatory/src/elements/helpers/tag.dart';
 import 'package:observatory/src/elements/helpers/uris.dart';
 import 'package:observatory/src/elements/nav/notify.dart';
 import 'package:observatory/src/elements/nav/refresh.dart';
 import 'package:observatory/src/elements/nav/top_menu.dart';
 import 'package:observatory/src/elements/nav/vm_menu.dart';
@@ skipping 116 matching lines @@
           }),
         new BRElement(),
         tree = new CpuProfileVirtualTreeElement(_vm, _progress.profile,
             queue: _r.queue, type: M.SampleProfileType.memory)
       ]);
     }
     children = content;
   }
 
   Future _request({bool forceFetch: false}) async {
-    for (Isolate isolate in vm.isolates) {
-      await isolate.invokeRpc("_collectAllGarbage", {});
+    // TODO: Is this casting correct?

[rmacnak, 2017/06/01 19:30:46]

This casting will work in practice in Observatory, but it isn't correct.
collectAllGarbage should be added to M.Isolate.

[devoncarew, 2017/06/01 20:11:54]

The issue here was more around the isolates getter: "The getter 'isolates' isn't
defined for the class 'VMRef'". It's using the 'vm' getter here - it should
perhaps just use the _vm field instead.

(updated this to use _vm)

+    for (M.Isolate isolate in (vm as M.VM).isolates) {
+      await (isolate as S.Isolate).invokeRpc("_collectAllGarbage", {});

[siva, 2017/06/01 19:26:32]

Not sure about this cast, according to Ryan the right fix is to have
_collectAllGarbage added to the Isolate interface.

[devoncarew, 2017/06/01 20:11:54]

The issue reported here was that M.IsolateRef doesn't define invokeRpc() -
that's defined down at the Isolate service object.

     }
     _progress = null;
     _progressStream = _profiles.get(_vm, _tag, forceFetch: forceFetch);
     _r.dirty();
     _progress = (await _progressStream.first).progress;
     _r.dirty();
     if (M.isSampleProcessRunning(_progress.status)) {
       _progress = (await _progressStream.last).progress;
       _r.dirty();
     }
   }
 
   Future _refresh(e) async {
     e.element.disabled = true;
     await _request(forceFetch: true);
     e.element.disabled = false;
   }
 }