Dafny generates invalid Java code

[sorawee]

method Main () 
{
  var x := 0;
  var z := (
    var y := 0;
    x
  );
}

generates the following Java code:

  public static void Main()
  {
    java.math.BigInteger _13_x = java.math.BigInteger.ZERO;
    _13_x = java.math.BigInteger.ZERO;
    java.math.BigInteger _14_z = java.math.BigInteger.ZERO;
    _14_z = dafny.Helpers.<java.math.BigInteger, java.math.BigInteger>Let(java.math.BigInteger.ZERO, _pat_let0_0 -> dafny.Helpers.<java.math.BigInteger, java.math.BigInteger>Let(_pat_let0_0, _15_y -> _13_x));
  }

which errors:

_System/__default.java:15: error: local variables referenced from a lambda expression must be final or effectively final