Enable strong-mode for pkg/kernel and fix some strong-mode warnings.

pkg/kernel/bin/dartk.dart

 #!/usr/bin/env dart
 // Copyright (c) 2016, 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.
 
 import 'dart:async';
 import 'dart:io';
 
 import 'batch_util.dart';
 
@@ skipping 244 matching lines @@
     return fail('Exactly one FILE should be given.');
   }
 
   var file = options.rest.single;
 
   checkIsFile(file, option: 'Input file');
 
   String format = options['format'] ?? defaultFormat();
   String outputFile = options['out'] ?? defaultOutput();
 
-  var customUriMappings = parseCustomUriMappings(options['url-mapping']);
+  List<String> urlMapping = options['url-mapping'] as List<String>;
+  var customUriMappings = parseCustomUriMappings(urlMapping);
   var repository = new Repository();
 
   Program program;
 
   var watch = new Stopwatch()..start();
   List<String> loadedFiles;
   Function getLoadedFiles;
   List errors = const [];
   TargetFlags targetFlags = new TargetFlags(strongMode: options['strong']);
   Target target = getTarget(options['target'], targetFlags);
@@ skipping 96 matching lines @@
 
   await ioFuture;
 
   if (shouldReportMetrics) {
     int flushTime = watch.elapsedMilliseconds - time;
     print('writer.flush_time = $flushTime ms');
   }
 
   return errors.length > 0 ? CompilerOutcome.Fail : CompilerOutcome.Ok;
 }