anchor.diff:    /// consists of. This includes the defining compilation unit and units
     /// included using the `part` directive.
     List<CompilationUnitElement> get units;
  -
  -  bool get isNonNullableByDefault;
   
     /// Return a list containing all of the imports that share the given [prefix],
     /// or an empty array if there are no such imports.
  
positive.code:   /// Return `true` if this library is part of the SDK.
    bool get isInSdk;
  
    /// Return a list containing the strongly connected component in the
    /// import/export graph in which the current library resides.
    List<LibraryElement> get libraryCycle;
  
negative.code:   LibraryElement get exportedLibrary;
  }
  
  /// An element that represents an extension.
  ///
  /// Clients may not extend, implement or mix-in this class.
  abstract class ExtensionElement implements Element {
    /// Return a list containing all of the accessors (getters and setters)
    /// declared in this extension.
    List<PropertyAccessorElement> get accessors;
  
    /// Return the type that is extended by this extension.
    DartType get extendedType;
  
    /// Return a list containing all of the methods declared in this extension.
    List<MethodElement> get methods;
  
    /// Return a list containing all of the type parameters declared by this
    /// extension.
    List<TypeParameterElement> get typeParameters;
  
    /// Return the element representing the getter with the given [name] that is
    /// declared in this extension, or `null` if this extension does not declare a
    /// getter with the given name.
    PropertyAccessorElement /*?*/ getGetter(String name);
  
    /// Return the element representing the method with the given [name] that is
    /// declared in this extension, or `null` if this extension does not declare a
    /// method with the given name.
    MethodElement /*?*/ getMethod(String name);
  
    /// Return the element representing the setter with the given [name] that is
    /// declared in this extension, or `null` if this extension does not declare a
    /// setter with the given name.
    PropertyAccessorElement /*?*/ getSetter(String name);
  }
  
  /// A field defined within a type.
  ///
  /// Clients may not extend, implement or mix-in this class.
  