Make removeDupes run in linear time

In the previous version, removeDupes used Array.splice, which ends up
moving the end of the array up in O(n) time, and makes the loop
effectively O(n^2). This version uses a dictionary for bookkeeping so
that duplicates can be removed in linear time.

Change-Id: I78e23b97e9cc932ee653823674fcc19eb90be342

appinventor/blocklyeditor/src/connection_db.js

@@ -83,10 +83,12 @@ Blockly.ConnectionDB.prototype.removeConnection_ = function(connection) {
  * O(n) removal of duplicate connections.
  */
 Blockly.ConnectionDB.prototype.removeDupes = function() {
-  for (var i = 0; i < this.length - 1; i++) {
-    if (this[i] == this[i+1]) {
-      this.splice(i, 1);
-      i--;
-    }
+  var map = {};
+  for (var i = 0; i < this.length; i++) {
+    var conn = this[i],
+      key = conn.getSourceBlock().id + ' ' + conn.x_ + ' ' + conn.y_;
+    if (!map[key]) map[key] = conn;
   }
+  this.splice(0, this.length);
+  Array.prototype.push.apply(this, Object.values(map));
 };