anchor.diff:      Expression expression = parseRelationalExpression('x as Function');
       expect(expression, isNotNull);
       assertNoErrors();
  -    expect(expression, new isInstanceOf<AsExpression>());
  -    AsExpression asExpression = expression;
  +    var asExpression = expression as AsExpression;
       expect(asExpression.expression, isNotNull);
       expect(asExpression.asOperator, isNotNull);
       expect(asExpression.type, new isInstanceOf<TypeName>());
  
positive.code:     Expression expression = parseConstExpression('const <A> []');
      expect(expression, isNotNull);
      assertNoErrors();
      expect(expression, new isInstanceOf<ListLiteral>());
      ListLiteral literal = expression;
      expect(literal.constKeyword, isNotNull);
      expect(literal.typeArguments, isNotNull);
      expect(literal.leftBracket, isNotNull);
  
negative.code:     Expression expression = parsePrimaryExpression('<A, B>{}');
      expect(expression, isNotNull);
      assertNoErrors();
      var literal = expression as MapLiteral;
      expect(literal.typeArguments, isNotNull);
      expect(literal.typeArguments.arguments, hasLength(2));
    }
  