Support tab completion of line:col in the debugger.

runtime/observatory/lib/src/elements/debugger.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library debugger_page_element;
 
 import 'dart:async';
 import 'dart:html';
 import 'observatory_element.dart';
 import 'package:observatory/app.dart';
@@ skipping 2303 matching lines @@
 @CustomTag('debugger-input')
 class DebuggerInputElement extends ObservatoryElement {
   @published Isolate isolate;
   @published String text = '';
   @observable ObservatoryDebugger debugger;
   @observable bool busy = false;
   @observable String modalPrompt = null;
   var modalCallback = null;
 
   void enterMode(String prompt, callback) {
-    assert(prompt == null);
+    assert(modalPrompt == null);
     modalPrompt = prompt;
     modalCallback = callback;
   }
 
   void exitMode() {
-    assert(prompt != null);
+    assert(modalPrompt != null);
     modalPrompt = null;
     modalCallback = null;
   }
 
   @override
   void ready() {
     super.ready();
     var textBox = $['textBox'];
     textBox.select();
     textBox.onKeyDown.listen((KeyboardEvent e) {
@@ skipping 113 matching lines @@
         }
       });
   }
 
   void focus() {
     $['textBox'].focus();
   }
 
   DebuggerInputElement.created() : super.created();
 }