Add a flag to control the backend.

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 48 matching lines @@
   List<String> options = new List<String>();
   bool explicitOut = false;
   bool wantHelp = false;
 
   passThrough(String argument) => options.add(argument);
 
   List<String> arguments = <String>[];
   List<OptionHandler> handlers = <OptionHandler>[
     new OptionHandler('--throw-on-error', (_) => throwOnError = true),
     new OptionHandler('--suppress-warnings', (_) => showWarnings = false),
+    new OptionHandler('--output-type=dart|--output-type=js', passThrough),
     new OptionHandler('--verbose|-v', (_) => verbose = true),
     new OptionHandler('--library-root=.+', (String argument) {
       String path = nativeToUriPath(extractParameter(argument));
       if (!path.endsWith("/")) path = "$path/";
       libraryRoot = cwd.resolve(path);
     }),
     new OptionHandler('--out=.+|-o.+', (String argument) {
       explicitOut = true;
       out = cwd.resolve(nativeToUriPath(extractParameter(argument)));
     }),
@@ skipping 152 matching lines @@
 
   -c, --enable-checked-mode, --checked
     Insert runtime type checks and enable assertions (checked mode).
 
   -h, /h, /?, --help
     Display this message (add -v for information about all options).
 
   -v, --verbose
     Display verbose information.
 
+  --output-type=<value>
+    Output either JavaScript code (if value is 'js') or Dart code (if value is 'dart').

[ahe, 2012/05/22 10:36:07]

FYI: long line (please stay below 80 characters in the future).

I'm editing this message and noticed it, and I'll fix it.

+    Default value is 'js'.
+
   --suppress-warnings
     Do not display any warnings.
 
   --no-colors
     Do not add colors to diagnostic messages.
 
 The following options are only used for compiler development and may
 be removed in a future version:
   --throw-on-error
     Throw an exception if a compile-time error is detected.
@@ skipping 30 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.
     }
   }
 }