Add no-op stub .neighbors() for headless Connection.

core/block.js

@@ -330,6 +330,7 @@ Blockly.Block.prototype.lastConnectionInStack_ = function() {
  * connected should not coincidentally line up on screen.
  * @private
  */
+// TODO: Refactor to return early in headless mode.
 Blockly.Block.prototype.bumpNeighbours_ = function() {
   if (!this.workspace) {
     return;  // Deleted block.

core/connection.js

@@ -622,6 +622,22 @@ Blockly.Connection.prototype.getShadowDom = function() {
   return this.shadowDom_;
 };
 
+/**
+ * Find all nearby compatible connections to this connection.
+ * Type checking does not apply, since this function is used for bumping.
+ *
+ * Headless configurations (the default) do not have neighboring connection,
+ * and always return an empty list (the default).
+ * {@link Blockly.RenderedConnection} overrides this behavior with a list
+ * computed from the rendered positioning.
+ * @param {number} maxLimit The maximum radius to another connection.
+ * @return {!Array.<!Blockly.Connection>} List of connections.
+ * @private
+ */
+Blockly.Connection.prototype.neighbours_ = function(/* maxLimit */) {
+  return [];
+};
+
 /**
  * This method returns a string describing this Connection in developer terms
  * (English only). Intended to on be used in console logs and errors.