diff --git a/runtime/observatory/lib/object_graph.dart b/runtime/observatory/lib/object_graph.dart
index 3ccc6c349b2..044f123830e 100644
--- a/runtime/observatory/lib/object_graph.dart
+++ b/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);
     }
diff --git a/runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart b/runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart
index 9b90eaa8a03..130d56ce2fc 100644
--- a/runtime/observatory/lib/src/heap_snapshot/heap_snapshot.dart
+++ b/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(