Make Enqueuer a pure interface.

pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart

Index: pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
index d903f99942dbe52db2635abc188e13bfca35c335..c1da15b54ffb170792edcc7db628532f816d4417 100644
--- a/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
@@ -2148,7 +2148,7 @@ function(originalDescriptor, name, holder, isStatic, globalFunctionsAccess) {
   void invalidateCaches() {
     if (!compiler.options.hasIncrementalSupport) return;
     if (cachedElements.isEmpty) return;
-    for (Element element in compiler.enqueuer.codegen.newlyEnqueuedElements) {
+    for (Element element in backend.codegenEnqueuer.newlyEnqueuedElements) {
       if (element.isInstanceMember) {
         cachedClassBuilders.remove(element.enclosingClass);