Add a flag to control the backend.

lib/compiler/implementation/dart2js.dart

Index: lib/compiler/implementation/dart2js.dart
diff --git a/lib/compiler/implementation/dart2js.dart b/lib/compiler/implementation/dart2js.dart
index c455e13dfdfce796f3c40ddbd5f75f10c1bed869..9743324b73c9e283b05862d8fb076a336772f241 100644
--- a/lib/compiler/implementation/dart2js.dart
+++ b/lib/compiler/implementation/dart2js.dart
@@ -66,6 +66,7 @@ void compile(List<String> argv) {
   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));
@@ -238,6 +239,10 @@ Supported 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.