Support tab completion of line:col in the debugger.

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

Index: runtime/observatory/lib/src/elements/debugger.dart
diff --git a/runtime/observatory/lib/src/elements/debugger.dart b/runtime/observatory/lib/src/elements/debugger.dart
index 472335858137e2108a680ab5cf4988d9e86e1b85..10d5a677610cb84ab0c2ca2ae9832b99499e4ebd 100644
--- a/runtime/observatory/lib/src/elements/debugger.dart
+++ b/runtime/observatory/lib/src/elements/debugger.dart
@@ -2321,13 +2321,13 @@ class DebuggerInputElement extends ObservatoryElement {
   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;
   }