anchor.diff:  /// Clients may not extend, implement or mix-in this class.
   abstract class Literal implements Expression {}
   
  -/// A literal expression used as a pattern.
  -///
  -///    literalPattern ::=
  -///        [BooleanLiteral]
  -///        [DoubleLiteral]
  -///        [IntegerLiteral]
  -///        [NullLiteral]
  -///        [StringLiteral]
  -///
  -/// Clients may not extend, implement or mix-in this class.
  -@experimental
  -abstract class LiteralPattern implements DartPattern {
  -  /// Return the literal expression being used as a pattern.
  -  Literal get expression;
  -}
  -
   /// A single key/value pair in a map literal.
   ///
   ///    mapLiteralEntry ::=
  
positive.code:   /// Return the value to which the value of the declared variable will be
    /// compared, or `null` if the condition does not include an equality test.
    StringLiteral? get value;
  }
  
  /// A constant pattern.
  ///
  ///    constantPattern ::=
  ///        [Identifier]
  ///
  /// Clients may not extend, implement or mix-in this class.
  @experimental
  abstract class ConstantPattern implements DartPattern {
    /// The expression referencing the constant being matched.
    Expression get identifier;
  }
  
  /// A constructor declaration.
  
negative.code: 
    /// Return the semicolon terminating the statement.
    Token? get semicolon;
  }
  
  /// An expression being used as a pattern.
  ///
  /// The only expressions that can be validly used as a pattern are
  /// - `bool` literals
  /// - `double` literals
  /// - `int` literals
  /// - `null` literals
  /// - `String` literals
  /// - references to constant variables.
  ///
  /// This node is also used to recover from cases where a different kind of
  /// expression is used as a pattern, so clients need to handle the case where
  /// the expression is not one of the valid alternatives.
  ///
  ///    expressionPattern ::=
  ///        [Expression]
  ///
  /// Clients may not extend, implement or mix-in this class.
  @experimental
  abstract class ExpressionPattern implements DartPattern {
    /// Return the expression being used as a pattern.
    Expression get expression;
  }
  
  /// An expression used as a statement.
  