message: [Java][FlexBuffers] throwing exception for untyped fixed vectors
  Untyped fixed vectors are not supported in FlexBuffers. There
  was an assert to check for it, but on java, asserts are optional. This
  change converts the assertion into a runtime exception.
  Fixes
diff: @@ -502,7 +502,9 @@ public class FlexBuffersBuilder {
  * @return Value representing the created vector
  */
  private Value createVector(int key, int start, int length, boolean typed, boolean fixed, Value keys) {
  - assert (!fixed || typed); // typed=false, fixed=true combination is not supported.
  + if (fixed & !typed)
  + throw new UnsupportedOperationException("Untyped fixed vector is not supported");
  +
  // Figure out smallest bit width we can store this vector with.
  int bitWidth = Math.max(WIDTH_8, widthUInBits(length));
  int prefixElems = 1;
  