Fix for issue 5223 : Import 'dart:core' implicitly only if it has not been

runtime/vm/code_generator_test.cc

Index: runtime/vm/code_generator_test.cc
===================================================================
--- runtime/vm/code_generator_test.cc	(revision 12656)
+++ runtime/vm/code_generator_test.cc	(working copy)
@@ -268,6 +268,9 @@
   const String& lib_url = String::ZoneHandle(Symbols::New(url));
   Library& lib = Library::ZoneHandle(Library::New(lib_url));
   lib.Register();
+  Library& core_lib = Library::Handle(Library::CoreLibrary());
+  ASSERT(!core_lib.IsNull());
+  lib.AddImport(core_lib);
   return lib;
 }