Extensions on primitives are not working in the Playground -- Underscore property

[dbooth-boston]

In FHIR JSON, primitives are extended by prepending an underscore to the property name, and attaching the extension to that. For example:

  "birthDate": "1974-12-25",
  "_birthDate": {
    "extension": [
      {
        "url": "http://hl7.org/fhir/StructureDefinition/patient-birthTime",
        "valueDateTime": "1974-12-25T14:35:45-05:00"
      }
    ]
  },

But _birthDate is not recognized in the Playground. See example.

[dbooth-boston]

Closing as superseded by #158