Update FilteredNodeChildren.java

GROOVY-6122
Update NodeChildren.java

GROOVY-6122
GROOVY-6122: FilteredNodeChildren doesn't provide childNodes() iterator
GROOVY-6124: Array slice not compatible with @CompileStatic
   * fixed method being called if array slice is used with @CompileStatic
   * fixed getAt optimization only for number types
   * fixed type inference of array slice