Crash in TypeVarType.serialize() when extracting dict().get as a variable #2804

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/local/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/Users/guido/src/mypy/mypy/__main__.py", line 5, in <module>
    main(None)
  File "/Users/guido/src/mypy/mypy/main.py", line 42, in main
    res = type_check_only(sources, bin_dir, options)
  File "/Users/guido/src/mypy/mypy/main.py", line 86, in type_check_only
    options=options)
  File "/Users/guido/src/mypy/mypy/build.py", line 183, in build
    dispatch(sources, manager)
  File "/Users/guido/src/mypy/mypy/build.py", line 1531, in dispatch
    process_graph(graph, manager)
  File "/Users/guido/src/mypy/mypy/build.py", line 1768, in process_graph
    process_stale_scc(graph, scc)
  File "/Users/guido/src/mypy/mypy/build.py", line 1856, in process_stale_scc
    graph[id].write_cache()
  File "/Users/guido/src/mypy/mypy/build.py", line 1512, in write_cache
    self.manager)
  File "/Users/guido/src/mypy/mypy/build.py", line 862, in write_cache
    data = tree.serialize()
  File "/Users/guido/src/mypy/mypy/nodes.py", line 271, in serialize
    'names': self.names.serialize(self._fullname),
  File "/Users/guido/src/mypy/mypy/nodes.py", line 2265, in serialize
    data[key] = value.serialize(fullname, key)
  File "/Users/guido/src/mypy/mypy/nodes.py", line 2211, in serialize
    data['node'] = self.node.serialize()
  File "/Users/guido/src/mypy/mypy/nodes.py", line 671, in serialize
    'type': None if self.type is None else self.type.serialize(),
  File "/Users/guido/src/mypy/mypy/types.py", line 837, in serialize
    'items': [t.serialize() for t in self.items()],
  File "/Users/guido/src/mypy/mypy/types.py", line 837, in <listcomp>
    'items': [t.serialize() for t in self.items()],
  File "/Users/guido/src/mypy/mypy/types.py", line 763, in serialize
    for t in self.arg_types],
  File "/Users/guido/src/mypy/mypy/types.py", line 763, in <listcomp>
    for t in self.arg_types],
  File "/Users/guido/src/mypy/mypy/types.py", line 1079, in serialize
    'items': [t.serialize() for t in self.items],
  File "/Users/guido/src/mypy/mypy/types.py", line 1079, in <listcomp>
    'items': [t.serialize() for t in self.items],
  File "/Users/guido/src/mypy/mypy/types.py", line 496, in serialize
    assert not self.id.is_meta_var()
AssertionError