Fix some observatory analysis issues.

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

Index: runtime/observatory/lib/src/elements/native_memory_profiler.dart
diff --git a/runtime/observatory/lib/src/elements/native_memory_profiler.dart b/runtime/observatory/lib/src/elements/native_memory_profiler.dart
index 2820afe90ab423032a7a402ad86d67a8c3394bcf..b144e8e68ef8cec389b85769b19a896c761644f1 100644
--- a/runtime/observatory/lib/src/elements/native_memory_profiler.dart
+++ b/runtime/observatory/lib/src/elements/native_memory_profiler.dart
@@ -7,6 +7,7 @@ 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';
@@ -143,8 +144,9 @@ class NativeMemoryProfileElement extends HtmlElement implements Renderable {
   }
 
   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);