Add a flag to control the backend.

lib/compiler/implementation/compiler.dart

Index: lib/compiler/implementation/compiler.dart
diff --git a/lib/compiler/implementation/compiler.dart b/lib/compiler/implementation/compiler.dart
index 33b542175b29b62ec1f9e99b385026fa29ef3dae..718cabd20ac7b67cf5e2cf7cf933fe1c62a86898 100644
--- a/lib/compiler/implementation/compiler.dart
+++ b/lib/compiler/implementation/compiler.dart
@@ -73,6 +73,10 @@ class JavaScriptBackend extends Backend {
   void assembleProgram() => emitter.assembleProgram();
 }
 
+class DartBackend extends Backend {
+  DartBackend(Compiler compiler) : super(compiler);
+}
+
 class Compiler implements DiagnosticListener {
   final Map<String, LibraryElement> libraries;
   int nextFreeClassId = 0;
@@ -149,7 +153,6 @@ class Compiler implements DiagnosticListener {
     validator = new TreeValidatorTask(this);
     resolver = new ResolverTask(this);
     checker = new TypeCheckerTask(this);
-    backend = new JavaScriptBackend(this);

[Anton Muhin, 2012/05/21 15:22:08]

I am somewhat concerned that we create a compiler w/o backend now.

Other options I can see: pass options into compiler, pass special flag or a
closure into compiler.

[ahe, 2012/05/21 15:38:55]

I agree. How about adding one more optional argument to this constructor. This
could be a boolean that says which backend to use.

     enqueuer = new EnqueueTask(this);
     tasks = [scanner, dietParser, parser, resolver, checker,
              constantHandler, enqueuer];