Throw when calling abstract methods; fix abstract return types

src/ol/util.js

@@ -2,6 +2,15 @@
  * @module ol/util
  */
 
+/**
+ * @return {?} Any return.
+ */
+export function abstract() {
+  return /** @type {?} */ ((function() {
+    throw new Error('Unimplemented abstract method.');
+  })());
+}
+
 /**
  * Inherit the prototype methods from one constructor into another.
  *