anchor.diff:  }
   
   /// Assign a local variable or function parameter.
  +///
  +/// Evaluates to the value of [value].
   class VariableSet extends Expression {
     VariableDeclaration variable;
     Expression value;
  
positive.code: /// Expression of form `super.field = value`.
  ///
  /// This may invoke a setter or assign a field.
  class SuperPropertySet extends Expression {
    Name name;
    Expression value;
  
negative.code: }
  
  /// Directly assign a field, or call a setter.
  ///
  /// Evaluates to the value of [value].
  class DirectPropertySet extends Expression {
    Expression receiver;
    Member target;
  