Remove 'id' from XML unless realtime is enabled.

core/xml.js

@@ -62,7 +62,10 @@ Blockly.Xml.workspaceToDom = function(workspace) {
 Blockly.Xml.blockToDom_ = function(block) {
   var element = goog.dom.createDom('block');
   element.setAttribute('type', block.type);
-  element.setAttribute('id', block.id);
+  if (Blockly.Realtime.isEnabled()) {
+    // Only used by realtime.
+    element.setAttribute('id', block.id);
+  }
   if (block.mutationToDom) {
     // Custom data for an advanced block.
     var mutation = block.mutationToDom();
@@ -297,6 +300,7 @@ Blockly.Xml.domToBlockHeadless_ =
   }
   var id = xmlBlock.getAttribute('id');
   if (opt_reuseBlock && id) {
+    // Only used by realtime.
     block = Blockly.Block.getById(id, workspace);
     // TODO: The following is for debugging.  It should never actually happen.
     if (!block) {
@@ -441,7 +445,7 @@ Blockly.Xml.domToBlockHeadless_ =
   }
   // Give the block a chance to clean up any initial inputs.
   if (block.validate) {
-    block.validate.call(block);
+    block.validate();
   }
   return block;
 };