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.
* Commented out error for when calling `a.setParent(null)` if a is connected to 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`).
* Fixed lint errors.

core/block.js

@@ -747,7 +747,7 @@ Blockly.Block.prototype.setParent = function(newParent) {
 
   // Check that block is connected to new parent if new parent is not null.
   //    After todo is addressed, throw error if block is not disconnected from
-  //    superior block when setting newParent to null. 
+  //    superior block when setting newParent to null.
   var connection = this.previousConnection || this.outputConnection;
   var isConnected = !!(connection && connection.targetBlock());
 
@@ -756,8 +756,8 @@ Blockly.Block.prototype.setParent = function(newParent) {
   } else if (!isConnected && newParent) {
     throw Error('Block not connected to new parent.');
   } else if (isConnected && !newParent) {
-    throw Error('Cannot set parent to null while block is still connected to'
-        + ' superior block.');
+    throw Error('Cannot set parent to null while block is still connected to' +
+        ' superior block.');
   }
 
   if (this.parentBlock_) {