Make .enclosingClass always return the declaration

pkg/compiler/lib/src/ssa/builder.dart

Index: pkg/compiler/lib/src/ssa/builder.dart
diff --git a/pkg/compiler/lib/src/ssa/builder.dart b/pkg/compiler/lib/src/ssa/builder.dart
index b6ef7c1ee7e58b670f82ce3436fba59f178f192a..8fe59bd9ac6517c84bb778e7b816720c3ae1dc71 100644
--- a/pkg/compiler/lib/src/ssa/builder.dart
+++ b/pkg/compiler/lib/src/ssa/builder.dart
@@ -3993,10 +3993,10 @@ class SsaBuilder extends ast.Visitor
       bool isLength = selector.isGetter && selector.name == "length";
       if (isLength || selector.isIndex) {
         return closedWorld.isSubtypeOf(
-            element.enclosingClass.declaration, helpers.jsIndexableClass);
+            element.enclosingClass, helpers.jsIndexableClass);
       } else if (selector.isIndexSet) {
-        return closedWorld.isSubtypeOf(element.enclosingClass.declaration,
-            helpers.jsMutableIndexableClass);
+        return closedWorld.isSubtypeOf(
+            element.enclosingClass, helpers.jsMutableIndexableClass);
       } else {
         return false;
       }