Allow flat coordinates for Polygon constructor

src/ol/format/GMLBase.js

@@ -444,7 +444,6 @@ GMLBase.prototype.readPolygon = function(node, objectStack) {
   const flatLinearRings = pushParseAndPop([null],
     this.FLAT_LINEAR_RINGS_PARSERS_, node, objectStack, this);
   if (flatLinearRings && flatLinearRings[0]) {
-    const polygon = new Polygon(null);
     const flatCoordinates = flatLinearRings[0];
     const ends = [flatCoordinates.length];
     let i, ii;
@@ -452,8 +451,7 @@ GMLBase.prototype.readPolygon = function(node, objectStack) {
       extend(flatCoordinates, flatLinearRings[i]);
       ends.push(flatCoordinates.length);
     }
-    polygon.setFlatCoordinates(GeometryLayout.XYZ, flatCoordinates, ends);
-    return polygon;
+    return new Polygon(flatCoordinates, GeometryLayout.XYZ, ends);
   } else {
     return undefined;
   }