Use JS backend in serialization_analysis_test

pkg/compiler/lib/src/elements/modelx.dart

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index 121a88214fe2fb52ce9177beeffbb362505724de..cfc0dcbdaa55e255a158a7a80e5d841b4ef5739a 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -208,18 +208,6 @@ abstract class ElementX extends Element with ElementCommon {
     return null;
   }
 
-  ClassElement get contextClass {
-    ClassElement cls;
-    for (Element e = this; e != null; e = e.enclosingElement) {
-      if (e.isClass) {
-        // Record [e] instead of returning it directly. We need the last class
-        // in the chain since the first classes might be closure classes.
-        cls = e.declaration;
-      }
-    }
-    return cls;
-  }
-
   /**
    * Creates the scope for this element.
    */