Fix error conditions for setParent google#4989

* Error now thrown when calling `a.setParent(b)` on non-null b if the output/previous connection of a is not connected to an input/next connection of b without removing block from old parent's child list.
* Error now thrown when calling `a.setParent(null)` if a is connected to a superior block.
* Adjusted comment to reflect that blocks were no longer being disconnected in this method.
* Also changed == to === per google#4924 (`newParent` and `this.parentBlock_` must both be instances of `Blockly.Block`).

core/block.js

@@ -755,11 +755,9 @@ Blockly.Block.prototype.setParent = function(newParent) {
     throw Error('Block connected to superior one that is not new parent.');
   } else if (!isConnected && newParent) {
     throw Error('Block not connected to new parent.');
-  } else if (connection && !newParent) {
-    // TODO #4989: Clean up code so that error can be thrown in this case 
-    //     without breaking things.
-    // throw Error('Cannot set parent to null while block is still connected
-    //    to superior block.');
+  } else if (isConnected && !newParent) {
+    throw Error('Cannot set parent to null while block is still connected to'
+        + ' superior block.');
   }
 
   if (this.parentBlock_) {