Add RPC and UI for reachable size of an object / all instances of a class.

runtime/observatory/lib/src/service/object.dart

Index: runtime/observatory/lib/src/service/object.dart
diff --git a/runtime/observatory/lib/src/service/object.dart b/runtime/observatory/lib/src/service/object.dart
index f7775c16a6209b2e95aafaf84a5e50efe0e82027..8f5f7a40fd9a0eabf8e36847fbf9387a741219c4 100644
--- a/runtime/observatory/lib/src/service/object.dart
+++ b/runtime/observatory/lib/src/service/object.dart
@@ -1688,6 +1688,13 @@ class Isolate extends ServiceObjectOwner with Coverage {
     return invokeRpc('evaluateInFrame', params);
   }
 
+  Future<ServiceObject> getReachableSize(ServiceObject target) {
+    Map params = {
+      'targetId': target.id,
+    };
+    return invokeRpc('_getReachableSize', params);
+  }
+
   Future<ServiceObject> getRetainedSize(ServiceObject target) {
     Map params = {
       'targetId': target.id,