Introduce BackendImpact to separate enqueueing from data.

pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart

Index: pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
diff --git a/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart b/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
index 00394a59b95528fae6efa9a098633ac368922db0..e345a7c3b08426259622eca124ecb7ea1e061602 100644
--- a/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
+++ b/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
@@ -74,11 +74,7 @@ class CustomElementsAnalysis {
     joinFor(enqueuer).instantiatedClasses.add(classElement);
   }
 
-  void registerTypeLiteral(DartType type, Registry registry) {
-    assert(registry.isForResolution);
-    // In codegen we see the TypeConstants instead.
-    if (!registry.isForResolution) return;
-
+  void registerTypeLiteral(DartType type) {
     if (type.isInterfaceType) {
       // TODO(sra): If we had a flow query from the type literal expression to
       // the Type argument of the metadata lookup, we could tell if this type