Fix a bug in instance canonicalization and add debug mode checks that we are not missing any fields.

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 24044)
+++ runtime/vm/object.cc	(working copy)
@@ -9202,6 +9202,28 @@
 }
 
 
+#if defined(DEBUG)
+class CheckForPointers : public ObjectPointerVisitor {
+ public:
+  explicit CheckForPointers(Isolate* isolate)
+      : ObjectPointerVisitor(isolate), has_pointers_(false) {}
+
+  bool has_pointers() const { return has_pointers_; }
+
+  void VisitPointers(RawObject** first, RawObject** last) {
+    if (first != last) {
+      has_pointers_ = true;
+    }
+  }
+
+ private:
+  bool has_pointers_;
+
+  DISALLOW_COPY_AND_ASSIGN(CheckForPointers);
+};
+#endif  // DEBUG
+
+
 RawInstance* Instance::CheckAndCanonicalize(const char** error_str) const {
   ASSERT(!IsNull());
   if (this->IsCanonical()) {
@@ -9211,7 +9233,7 @@
   const Class& cls = Class::Handle(this->clazz());
   // TODO(srdjan): Check that predefined classes do not have fields that need
   // to be checked/canonicalized as well.
-  if ((cls.id() >= kNumPredefinedCids) || cls.IsArray()) {
+  if ((cls.id() >= kNumPredefinedCids) || IsArray()) {
     // Iterate over all fields, canonicalize numbers and strings, expect all
     // other instances to be canonical otherwise report error (return
     // Instance::null()).
@@ -9238,6 +9260,13 @@
         }
       }
     }
+  } else {
+#if defined(DEBUG)
+    // Make sure that we are not missing any fields.
+    CheckForPointers has_pointers(Isolate::Current());
+    this->raw()->VisitPointers(&has_pointers);
+    ASSERT(!has_pointers.has_pointers());
+#endif  // DEBUG
   }
   Array& constants = Array::Handle(cls.constants());
   const intptr_t constants_len = constants.Length();