Teach the Observatory about WeakProperties (ephemerons).

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index c7ef20ed235e10a0ae0afce94fdad8a893a6e717..394ce89c0c5faf78296ee4719e15c285e4f14c81 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -19483,7 +19483,20 @@ const char* WeakProperty::ToCString() const {
 
 
 void WeakProperty::PrintJSONImpl(JSONStream* stream, bool ref) const {
-  Instance::PrintJSONImpl(stream, ref);
+  JSONObject jsobj(stream);
+  PrintSharedInstanceJSON(&jsobj, ref);
+  ObjectIdRing* ring = Isolate::Current()->object_id_ring();
+  const intptr_t id = ring->GetIdForObject(raw());
+  jsobj.AddPropertyF("id", "objects/%" Pd "", id);
+
+  if (ref) {
+    return;
+  }
+
+  const Object& key_handle = Object::Handle(key());
+  jsobj.AddProperty("key", key_handle);
+  const Object& value_handle = Object::Handle(value());
+  jsobj.AddProperty("value", value_handle);
 }
 
 RawAbstractType* MirrorReference::GetAbstractTypeReferent() const {