Adds new apis Dart_GetField/Dart_SetField.

vm/object.h

Index: vm/object.h
===================================================================
--- vm/object.h	(revision 4977)
+++ vm/object.h	(working copy)
@@ -1747,6 +1747,8 @@
   RawClass* LookupClass(const String& name) const;
   RawObject* LookupLocalObject(const String& name) const;
   RawClass* LookupLocalClass(const String& name) const;
+  RawField* LookupLocalField(const String& name) const;
+  RawFunction* LookupLocalFunction(const String& name) const;
   RawLibraryPrefix* LookupLocalLibraryPrefix(const String& name) const;
   RawScript* LookupScript(const String& url) const;
   RawArray* LoadedScripts() const;
@@ -1771,7 +1773,7 @@
     raw_ptr()->native_entry_resolver_ = value;
   }
 
-  RawString* PrivateName(const char* name);
+  RawString* PrivateName(const String& name) const;
 
   void Register() const;