Parameterize backend

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index f6353790b591b93f3bebbb4a9327f03e6910937b..3594098f6d5574902b789e8cfd0180734c9c5771 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -549,6 +549,8 @@ abstract class Backend {
   void registerAsyncMarker(FunctionElement element,
                              Enqueuer enqueuer,
                              Registry registry) {}
+
+  EnqueueTask makeEnqueuer() => new EnqueueTask(compiler);
 }
 
 /// Backend callbacks function specific to the resolution phase.
@@ -1066,7 +1068,8 @@ abstract class Compiler implements DiagnosticListener {
             this.generateCodeWithCompileTimeErrors: false,
             this.testMode: false,
             api.CompilerOutput outputProvider,
-            List<String> strips: const []})
+            List<String> strips: const [],
+            Backend makeBackend(Compiler compiler)})
       : this.disableTypeInferenceFlag =
           disableTypeInferenceFlag || !emitJavaScript,
         this.analyzeOnly =
@@ -1098,7 +1101,9 @@ abstract class Compiler implements DiagnosticListener {
     globalDependencies =
         new CodegenRegistry(this, new TreeElementMapping(null));
 
-    if (emitJavaScript) {
+    if (makeBackend != null) {
+      backend = makeBackend(this);
+    } else if (emitJavaScript) {
       js_backend.JavaScriptBackend jsBackend =
           new js_backend.JavaScriptBackend(
               this, generateSourceMap: generateSourceMap,
@@ -1127,7 +1132,7 @@ abstract class Compiler implements DiagnosticListener {
       constants = backend.constantCompilerTask,
       deferredLoadTask = new DeferredLoadTask(this),
       mirrorUsageAnalyzerTask = new MirrorUsageAnalyzerTask(this),
-      enqueuer = new EnqueueTask(this),
+      enqueuer = backend.makeEnqueuer(),
       dumpInfoTask = new DumpInfoTask(this),
       reuseLibraryTask = new GenericTask('Reuse library', this),
     ];