Prevent the allInstances array from creating spurious references on future heap queries.

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

Index: runtime/observatory/lib/src/elements/heap_snapshot.dart
diff --git a/runtime/observatory/lib/src/elements/heap_snapshot.dart b/runtime/observatory/lib/src/elements/heap_snapshot.dart
index 0b1ffcfdc2c236bff55ae2417b9645451a73136e..0913e1ebdb257784f40a1b1f634caa57f11a01fe 100644
--- a/runtime/observatory/lib/src/elements/heap_snapshot.dart
+++ b/runtime/observatory/lib/src/elements/heap_snapshot.dart
@@ -353,7 +353,7 @@ class HeapSnapshotElement  extends HtmlElement implements Renderable {
       element.children[3].text = '';
       if (item is Iterable<M.HeapSnapshotClassInbound>) {
         element.children[4] = new SpanElement()..classes = const ['name']
-            ..text = '${item.length} Ingoing references';
+            ..text = '${item.length} Incoming references';
       } else {
         element.children[4] = new SpanElement()..classes = const ['name']
             ..text = '${item.length} Outgoing references';