[observatory] Add an "ownership" analysis for the heap snapshot.

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

Index: runtime/observatory/lib/src/models/objects/heap_snapshot.dart
diff --git a/runtime/observatory/lib/src/models/objects/heap_snapshot.dart b/runtime/observatory/lib/src/models/objects/heap_snapshot.dart
index aae5493e853f0ef29376eb8ea5a246df4e8c1d46..9250370aa8ef997009d592f23e1f75dfa55ac98b 100644
--- a/runtime/observatory/lib/src/models/objects/heap_snapshot.dart
+++ b/runtime/observatory/lib/src/models/objects/heap_snapshot.dart
@@ -14,6 +14,7 @@ abstract class HeapSnapshot {
   HeapSnapshotDominatorNode get dominatorTree;
   HeapSnapshotMergedDominatorNode get mergedDominatorTree;
   Iterable<HeapSnapshotClassReferences> get classReferences;
+  Iterable<HeapSnapshotOwnershipClass> get ownershipClasses;
 }
 
 abstract class HeapSnapshotDominatorNode {
@@ -55,3 +56,8 @@ abstract class HeapSnapshotClassOutbound {
   int get shallowSize;
   int get retainedSize;
 }
+
+abstract class HeapSnapshotOwnershipClass {
+  ClassRef get clazz;
+  int get size;
+}