Minor fix to ddbg when current isolate goes away.

tools/ddbg.dart

 // Copyright (c) 2012, 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.
 
 // Simple interactive debugger shell that connects to the Dart VM's debugger
 // connection port.
 
 import "dart:convert";
 import "dart:io";
 import "dart:async";
@@ skipping 1130 matching lines @@
     assert(isolate != null);
     if (isolate == mainIsolate) {
       mainIsolate = null;
       print("Main isolate ${isolate.id} has terminated.");
     } else {
       print("Isolate ${isolate.id} has terminated.");
     }
     if (isolate == currentIsolate) {
       currentIsolate = mainIsolate;
       if (currentIsolate == null && !targetIsolates.isEmpty) {
-        currentIsolate = targetIsolates.first;
+        currentIsolate = targetIsolates.values.first;
       }
       if (currentIsolate != null) {
         print("Setting current isolate to ${currentIsolate.id}.");
       } else {
         print("All isolates have terminated.");
       }
     }
   }
 }
 
@@ skipping 320 matching lines @@
       handleUncaughtError: debuggerError));
 
   zone.run(() {
       parseArgs(args);
       cmdo = new Commando(completer: debuggerCommandCompleter);
       cmdSubscription = cmdo.commands.listen(processCommand,
                                              onError: processError,
                                              onDone: processDone);
     });
 }