Fix 'toplist by retained memory' in observatory

Prior to this fix I get errors like

```
TypeError: Instance of 'b2<bQ>': type 'b2<bQ>' is not a subtype of type 'List<bQ>'

StackTrace:

main.dart.js 1631:3    Object.m
main.dart.js 2001:9    Object.p
[...]
```

Change-Id: I2fc6329c05cb76bff76929aeee1eca3954fbc0a9
Reviewed-on: https://dart-review.googlesource.com/70501
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

runtime/observatory/lib/object_graph.dart

@@ -452,7 +452,7 @@ class ObjectGraph {
         new List<ObjectVertex>.from(vertices.where((u) => !u.isRoot));
     _mostRetained.sort((u, v) => v.retainedSize - u.retainedSize);
 
-    var result = _mostRetained;
+    Iterable<ObjectVertex> result = _mostRetained;
     if (classId != null) {
       result = result.where((u) => u.vmCid == classId);
     }

runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart

@@ -116,7 +116,7 @@ class HeapSnapshot implements M.HeapSnapshot {
 
   List<Future<S.ServiceObject>> getMostRetained(S.Isolate isolate,
       {int classId, int limit}) {
-    var result = [];
+    List<Future<S.ServiceObject>> result = <Future<S.ServiceObject>>[];
     for (ObjectVertex v
         in graph.getMostRetained(classId: classId, limit: limit)) {
       result.add(