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

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 24056)
+++ runtime/vm/object.h	(working copy)
@@ -3531,6 +3531,9 @@
   // Returns error in error_str, pass NULL if an error cannot occur.
   virtual RawInstance* CheckAndCanonicalize(const char** error_str) const;
 
+  // Returns true if all fields are OK for canonicalization.
+  virtual bool CheckAndCanonicalizeFields(const char** error_str) const;
+
   RawObject* GetField(const Field& field) const {
     return *FieldAddr(field);
   }
@@ -4984,6 +4987,9 @@
     return RoundedAllocationSize(sizeof(RawArray) + (len * kBytesPerElement));
   }
 
+  // Returns true if all elements are OK for canonicalization.
+  virtual bool CheckAndCanonicalizeFields(const char** error_str) const;
+
   // Make the array immutable to Dart code by switching the class pointer
   // to ImmutableArray.
   void MakeImmutable() const;