Don't use fatal errors in message_kind_test.dart.

dart/tests/compiler/dart2js/memory_compiler.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.test.memory_compiler;
 
 import 'memory_source_file_helper.dart';
 
 import '../../../sdk/lib/_internal/compiler/implementation/dart2jslib.dart'
        show NullSink;
@@ skipping 191 matching lines @@
     cachedCompiler.parser = null;
     cachedCompiler.patchParser = null;
     cachedCompiler.libraryLoader = null;
     cachedCompiler.validator = null;
     cachedCompiler.resolver = null;
     cachedCompiler.closureToClassMapper = null;
     cachedCompiler.checker = null;
     cachedCompiler.irBuilder = null;
     cachedCompiler.typesTask = null;
     cachedCompiler.backend = null;
-    cachedCompiler.enqueuer = null;
+    // Don't null out the enqueuer as it prevents us from using cachedCompiler
+    // more than once.
     cachedCompiler.deferredLoadTask = null;
     cachedCompiler.mirrorUsageAnalyzerTask = null;
     cachedCompiler.dumpInfoTask = null;
     cachedCompiler.buildId = null;
   }
   return compiler;
 }
 
 Future<MirrorSystem> mirrorSystemFor(Map<String,String> memorySourceFiles,
                                      {DiagnosticHandler diagnosticHandler,
                                       List<String> options: const [],
                                       bool showDiagnostics: true}) {
   Uri libraryRoot = Uri.base.resolve('sdk/');
   Uri packageRoot = Uri.base.resolve('${Platform.packageRoot}/');
   Uri script = Uri.base.resolveUri(Platform.script);
 
   var provider = new MemorySourceFileProvider(memorySourceFiles);
   var handler =
       createDiagnosticHandler(diagnosticHandler, provider, showDiagnostics);
 
   List<Uri> libraries = <Uri>[];
   memorySourceFiles.forEach((String path, _) {
     libraries.add(new Uri(scheme: 'memory', path: path));
   });
 
   return analyze(libraries, libraryRoot, packageRoot,
                  provider, handler, options);
 }