Check that external references are registered in the serializer.

tools/presubmit.py

Index: tools/presubmit.py
diff --git a/tools/presubmit.py b/tools/presubmit.py
index c8d3a714070707e032d9434e6fff554660b4d151..cbc21f38945db7ca06336225b3b8958cff961c23 100755
--- a/tools/presubmit.py
+++ b/tools/presubmit.py
@@ -423,6 +423,12 @@ def CheckGeneratedRuntimeTests(workspace):
   return code == 0
 
 
+def CheckExternalReferenceRegistration(workspace):
+  code = subprocess.call(
+      [sys.executable, join(workspace, "tools", "external-reference-check.py")])
+  return code == 0
+
+
 def GetOptions():
   result = optparse.OptionParser()
   result.add_option('--no-lint', help="Do not run cpplint", default=False,
@@ -442,6 +448,7 @@ def Main():
         "two empty lines between declarations check..."
   success = SourceProcessor().Run(workspace) and success
   success = CheckGeneratedRuntimeTests(workspace) and success
+  success = CheckExternalReferenceRegistration(workspace) and success
   if success:
     return 0
   else: