Rework isClosed with added tests

src/ol/geom/flat/topologyflatgeom.js

@@ -0,0 +1,20 @@
+goog.provide('ol.geom.flat.topology');
+
+goog.require('ol.geom.flat.area');
+
+/**
+ * Check if the linestring is a boundary.
+ * @param {Array.<number>} flatCoordinates Flat coordinates.
+ * @param {number} offset Offset.
+ * @param {number} end End.
+ * @param {number} stride Stride.
+ * @return {boolean} The linestring is a boundary.
+ */
+ol.geom.flat.topology.lineStringIsClosed = function(flatCoordinates, offset, end, stride) {
+  var lastCoord = end - stride;
+  if (flatCoordinates[offset] === flatCoordinates[lastCoord] &&
+      flatCoordinates[offset + 1] === flatCoordinates[lastCoord + 1] && (end - offset) / stride > 3) {
+    return !!ol.geom.flat.area.linearRing(flatCoordinates, offset, end, stride);
+  }
+  return false;
+};