anchor.diff:    }
   
     Token parseSingleLiteralString(Token token) {
  +    assert(identical(token.kind, STRING_TOKEN));
       listener.beginLiteralString(token);
       // Parsing the prefix, for instance 'x of 'x${id}y${id}z'
       token = token.next;
  
positive.code:     }
    }
  
    Token parseLabel(Token token) {
      token = parseIdentifier(token, IdentifierContext.labelDeclaration);
      Token colon = token;
      token = expect(':', token);
  
negative.code:     }
    }
  
    /// ```
    /// yieldStatement:
    ///   'yield' expression? ';'
    /// ;
    /// ```
    Token parseYieldStatement(Token token) {
      assert(identical('yield', token.stringValue));
      Token begin = token;
  