Use JS backend in serialization_analysis_test

pkg/compiler/lib/src/native/enqueue.dart

Index: pkg/compiler/lib/src/native/enqueue.dart
diff --git a/pkg/compiler/lib/src/native/enqueue.dart b/pkg/compiler/lib/src/native/enqueue.dart
index 738a304641018ce9b50db7546c5264f547f9a90b..f9c113a3183c6f795db64cea1865232afc9c21ae 100644
--- a/pkg/compiler/lib/src/native/enqueue.dart
+++ b/pkg/compiler/lib/src/native/enqueue.dart
@@ -236,7 +236,18 @@ abstract class NativeEnqueuerBase implements NativeEnqueuer {
    * Returns the source string of the class named in the extends clause, or
    * `null` if there is no extends clause.
    */
-  String findExtendsNameOfClass(BaseClassElementX classElement) {
+  String findExtendsNameOfClass(ClassElement classElement) {
+    if (classElement.isResolved) {
+      ClassElement superClass = classElement.superclass;
+      while (superClass != null) {
+        if (!superClass.isUnnamedMixinApplication) {
+          return superClass.name;
+        }
+        superClass = superClass.superclass;
+      }
+      return null;
+    }
+
     //  "class B extends A ... {}"  --> "A"
     //  "class B extends foo.A ... {}"  --> "A"
     //  "class B<T> extends foo.A<T,T> with M1, M2 ... {}"  --> "A"
@@ -368,7 +379,7 @@ abstract class NativeEnqueuerBase implements NativeEnqueuer {
     flushing = false;
   }
 
-  processClass(BaseClassElementX classElement, cause) {
+  processClass(ClassElement classElement, cause) {
     // TODO(ahe): Fix this assertion to work in incremental compilation.
     assert(compiler.hasIncrementalSupport ||
            !registeredClasses.contains(classElement));