Fix side effect issues with predefined names and lazy contexts.

jedi/evaluate/context.py

@@ -99,9 +99,17 @@ class LazyTreeContext(AbstractLazyContext):
     def __init__(self, context, node):
         super(LazyTreeContext, self).__init__(node)
         self._context = context
+        # We need to save the predefined names. It's an unfortunate side effect
+        # that needs to be tracked otherwise results will be wrong.
+        self._predefined_names = dict(context.predefined_names)
 
     def infer(self):
-        return self._context.eval_node(self.data)
+        old, self._context.predefined_names = \
+            self._context.predefined_names, self._predefined_names
+        try:
+            return self._context.eval_node(self.data)
+        finally:
+            self._context.predefined_names = old
 
 
 def get_merged_lazy_context(lazy_contexts):