Remove O(N) cell by msg-id lookup

IPython/html/static/notebook/js/codecell.js

@@ -105,6 +105,7 b' var IPython = (function (IPython) {'
         }
     };
 
+    CodeCell.msg_cells = {};
 
     CodeCell.prototype = new IPython.Cell();
 
@@ -317,7 +318,12 b' var IPython = (function (IPython) {'
         }
         var callbacks = this.get_callbacks();
         
+        var old_msg_id = this.last_msg_id;
         this.last_msg_id = this.kernel.execute(this.get_text(), callbacks, {silent: false, store_history: true});
+        if (old_msg_id) {
+            delete CodeCell.msg_cells[old_msg_id];
+        }
+        CodeCell.msg_cells[this.last_msg_id] = this;
     };
     
     /**

IPython/html/static/notebook/js/notebook.js

@@ -308,12 +308,8 b' var IPython = (function (IPython) {'
      * @return {Cell} Cell or null if no cell was found.
      */
     Notebook.prototype.get_msg_cell = function (msg_id) {
-        var cells = this.get_cells();
-        for (var cell_index in cells) {
-            if (cells[cell_index].last_msg_id == msg_id) {
-                var cell = this.get_cell(cell_index)
-                return cell;
-            }
+        if (IPython.CodeCell.msg_cells[msg_id] !== undefined) {
+            return IPython.CodeCell.msg_cells[msg_id];
         }
         return null;
     };