Do not add secondary interfaces implemented by parents.

lib/html/scripts/systemhtml.py

Index: lib/html/scripts/systemhtml.py
diff --git a/lib/html/scripts/systemhtml.py b/lib/html/scripts/systemhtml.py
index ab52d3c2be96f732d50d2893a6945c2f71281ce8..dd776674c531d9dbacbe9731e2ca9945cc050d98 100644
--- a/lib/html/scripts/systemhtml.py
+++ b/lib/html/scripts/systemhtml.py
@@ -554,11 +554,16 @@ class HtmlDartInterfaceGenerator(object):
     """
     def walk(parents):
       for parent in parents:
-        if IsDartCollectionType(parent.type.id):
-          result.append(parent.type.id)
+        parent_name = parent.type.id
+        if parent_name == 'EventTarget':
+          # Currently EventTarget is implemented as a mixin, not a proper
+          # super interface---ignore its members.
           continue
-        if self._database.HasInterface(parent.type.id):
-          parent_interface = self._database.GetInterface(parent.type.id)
+        if IsDartCollectionType(parent_name):
+          result.append(parent_name)
+          continue
+        if self._database.HasInterface(parent_name):
+          parent_interface = self._database.GetInterface(parent_name)
           result.append(parent_interface)
           walk(parent_interface.parents)