[ALL] `$ref` in root of defined schema not handled and causes error

Description

$ref in root of defined schema not handled and causes error

e.g a schema defined as the one below causes an error

...
definitions:
  Foo:
    $ref: '#/definitions/Bar'
  Bar:
    type: string
...

Swagger-codegen version

latest master, probably also happening in 3.0.0

Swagger declaration file content or url

...
definitions:
  Foo:
    $ref: '#/definitions/Bar'
  Bar:
    type: string
...

Steps to reproduce

generate javascript client code with spec containing snippet above

This scenario should be handled, or at least a proper "not supported" error should be provided; this applies to master / OAS2 but probably also to 3.0.0 / OAS3