Implement automatic loading of dart:core_native_fields library

vm/class_finalizer.cc

Index: vm/class_finalizer.cc
===================================================================
--- vm/class_finalizer.cc	(revision 1624)
+++ vm/class_finalizer.cc	(working copy)
@@ -999,6 +999,18 @@
   if (cls.is_const()) {
     CheckForLegalConstClass(cls);
   }
+  // Check to ensure we don't have classes with native fields in libraries
+  // which do not have a native resolver.
+  if (cls.num_native_fields() != 0) {
+    const Library& lib = Library::Handle(cls.library());
+    if (lib.native_entry_resolver() == NULL) {
+      const String& cls_name = String::Handle(cls.Name());
+      const String& lib_name = String::Handle(lib.url());
+      ReportError("class '%s' is trying to extend a native fields class,"
+                  "but library '%s' has no native resolvers",
+                  cls_name.ToCString(), lib_name.ToCString());
+    }
+  }
 }