The operator += is undefined for the argument type(s) CharSequence, String

The following code compiles with `javac`, but produces an error with jdt:
```java
public class JdtBug {
public static void main(String[] args) {
System.out.println("Hello, World!");
}

private static String convert(char[] seq) {
CharSequence res = "";
res += "";
return res.toString();
}
}
```