getSchemaKey = function(schema,key) {
  var schemaKey = key
    .replace(/\./g, '.properties.')
    .replace(/\[[0-9]*\]/g, '.items');
  var schemaDef = jsonform.util.getObjKey(schema, schemaKey, true);
  if (schemaDef && schemaDef.$ref) {
    throw new Error('JSONForm does not yet support schemas that use the ' +
      '$ref keyword. See: https://github.com/joshfire/jsonform/issues/54');
  }
  return schemaDef;
}
Retrieves the key definition from the given schema.

The key is identified by the path that leads to the key in the
structured object that the schema would generate. Each level is
separated by a '.'. Array levels are marked with []. For instance:
 foo.bar[].baz
... to retrieve the definition of the key at the following location
in the JSON schema (using a dotted path notation):
 foo.properties.bar.items.properties.baz

@function
@param {Object} schema The JSON schema to retrieve the key from
@param {String} key The path to the key, each level being separated
 by a dot and array items being flagged with [].
@return {Object} The key definition in the schema, null if not found.