anchor.diff:  
       test("test identifier expression", () {
   
  -        Program program = parseProgramChecked('foobar;');
  -
  -        expectNumStatements(program, 1);
  -
  -        expect(program.statements.first, new isInstanceOf<ExpressionStatement>());
  -        ExpressionStatement statement = program.statements.first;
  -
  +        ExpressionStatement statement = parseExpressionStatement('foobar;');
           expect(statement.expression, new isInstanceOf<Identifier>());
  +
           Identifier ident = statement.expression;
  -
           expect(ident.value, equals('foobar'));
           expect(ident.tokenLiteral(), equals('foobar'));
   
  
positive.code: 
      test("test literal integer expression", () {
  
          Program program = parseProgramChecked('5;');
  
          expectNumStatements(program, 1);
  
          expect(program.statements.first, new isInstanceOf<ExpressionStatement>());
          ExpressionStatement statement = program.statements.first;
  
          testIntegerLiteral(statement.expression, 5);
  
      });
  
negative.code: 
  void testPrefix(String input, String operator, Object expectedValue) {
  
      ExpressionStatement statement = parseExpressionStatement(input);
  
      expect(statement.expression, new isInstanceOf<PrefixExpression>());
      PrefixExpression expression = statement.expression;
  