Start implementing checked mode and tools support for using it.

lib/compiler/implementation/dart2js.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.
 
 #library('dart2js');
 
 #import('dart:io');
 #import('dart:uri');
 #import('dart:utf');
 
@@ skipping 28 matching lines @@
       if (!path.endsWith("/")) path = "$path/";
       libraryRoot = cwd.resolve(path);
     } else if (argument.startsWith('--out=')) {
       String path =
           nativeToUriPath(argument.substring(argument.indexOf('=') + 1));
       out = cwd.resolve(path);
     } else if ('--allow-mock-compilation' == argument) {
       options.add(argument);
     } else if ('--no-colors' == argument) {
       colors.enabled = false;
+    } else if ('--enable-checked-mode' == argument) {
+      options.add(argument);
     } else if (argument.startsWith('-')) {
       fail('Unknown option $argument.');
     } else {
       arguments.add(nativeToUriPath(argument));
     }
   }
   if (arguments.isEmpty()) {
     fail('No file to compile.');
   }
   if (arguments.length > 1) {
@@ skipping 96 matching lines @@
     } catch (var ignored) {
       print('Internal error: error while printing exception');
     }
     try {
       print(trace);
     } finally {
       exit(253); // 253 is recognized as a crash by our test scripts.
     }
   }
 }