code: function cloneRelayHandleSourceField(
    handleField: NormalizationLinkedHandle,
    selections: $ReadOnlyArray<NormalizationSelection>,
    variables: Variables,
  ): NormalizationLinkedField {
    const sourceField = selections.find(
      source =>
        source.kind === LINKED_FIELD &&
        source.name === handleField.name &&
        source.alias === handleField.alias &&
        areEqual(source.args, handleField.args),
    );
    invariant(
      sourceField && sourceField.kind === LINKED_FIELD,
      'cloneRelayHandleSourceField: Expected a corresponding source field for ' +
        'handle `%s`.',
      handleField.handle,
    );
    const handleKey = getHandleStorageKey(handleField, variables);
    const clonedField = {
      ...sourceField,
      args: null,
      name: handleKey,
      storageKey: handleKey,
    };
    return clonedField;
  }
docs: @private
  
  Creates a clone of the supplied `handleField` by finding the original linked
  field (on which the handle was declared) among the sibling `selections`, and
  copying its selections into the clone.