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