Fixed InstancedBufferGeometry.

src/core/InstancedBufferGeometry.js

@@ -16,7 +16,7 @@ function InstancedBufferGeometry() {
 InstancedBufferGeometry.prototype = Object.create( BufferGeometry.prototype );
 InstancedBufferGeometry.prototype.constructor = InstancedBufferGeometry;
 
-InstancedBufferGeometry.prototype.isBufferGeometry = true;
+InstancedBufferGeometry.prototype.isInstancedBufferGeometry = true;
 
 InstancedBufferGeometry.prototype.addGroup = function ( start, count, instances ) {