Support tab completion of line:col in the debugger.

runtime/observatory/lib/src/service/object.dart

Index: runtime/observatory/lib/src/service/object.dart
diff --git a/runtime/observatory/lib/src/service/object.dart b/runtime/observatory/lib/src/service/object.dart
index e65f5b76534045ecccc215c830d6b955232c7902..d38933fe6d2d99d83a0aaac84971a9b7ed2c2e6c 100644
--- a/runtime/observatory/lib/src/service/object.dart
+++ b/runtime/observatory/lib/src/service/object.dart
@@ -1506,7 +1506,9 @@ class Isolate extends ServiceObjectOwner with Coverage {
     } on ServerRpcException catch(e) {
       if (e.code == ServerRpcException.kCannotAddBreakpoint) {
         // Unable to set a breakpoint at the desired line.
-        script.getLine(line).possibleBpt = false;
+        if (script.loaded) {
+          script.getLine(line).possibleBpt = false;
+        }
       }
       rethrow;
     }