anchor.diff:  /// An export directive within a library.
   ///
   /// Clients may not extend, implement or mix-in this class.
  -abstract class ExportElement implements Element, UriReferencedElement {
  +abstract class ExportElement implements UriReferencedElement {
     /// Return a list containing the combinators that were specified as part of
     /// the export directive in the order in which they were specified.
     List<NamespaceCombinator> get combinators;
  
positive.code: /// An element representing a compilation unit.
  ///
  /// Clients may not extend, implement or mix-in this class.
  abstract class CompilationUnitElement implements Element, UriReferencedElement {
    /// Return a list containing all of the top-level accessors (getters and
    /// setters) contained in this compilation unit.
    List<PropertyAccessorElement> get accessors;
  
negative.code: /// A single import directive within a library.
  ///
  /// Clients may not extend, implement or mix-in this class.
  abstract class ImportElement implements UriReferencedElement {
    /// Return a list containing the combinators that were specified as part of
    /// the import directive in the order in which they were specified.
    List<NamespaceCombinator> get combinators;
  