Avoid spurious "The compiler crashed" messages in batch mode.

pkg/compiler/lib/src/dart2js.dart

 // Copyright (c) 2013, 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 dart2js.cmdline;
 
 import 'dart:async'
     show Future, EventSink;
 import 'dart:convert' show UTF8, LineSplitter;
 import 'dart:io'
@@ skipping 636 matching lines @@
     return;
   }
   internalMain(arguments);
 }
 
 var exitFunc = exit;
 var compileFunc = api.compile;
 
 Future<api.CompilationResult> internalMain(List<String> arguments) {
   Future onError(exception, trace) {
+    // If we are already trying to exit, just continue exiting.
+    if (exception == _EXIT_SIGNAL) throw exception;
+
     try {
       print('The compiler crashed: $exception');
     } catch (ignored) {
       print('The compiler crashed: error while printing exception');
     }
 
     try {
       if (trace != null) {
         print(trace);
       }
@@ skipping 50 matching lines @@
       } else if (exitCode == 253) {
         print(">>> TEST CRASH");
       } else {
         print(">>> TEST FAIL");
       }
       stderr.writeln(">>> EOF STDERR");
       subscription.resume();
     });
   });
 }