Make .enclosingClass always return the declaration

pkg/compiler/lib/src/universe/function_set.dart

Index: pkg/compiler/lib/src/universe/function_set.dart
diff --git a/pkg/compiler/lib/src/universe/function_set.dart b/pkg/compiler/lib/src/universe/function_set.dart
index 385e89cbaad2e1f97ef0c7ab1e71c3b6c2a4903c..3dc566ab5b6accbad4320a0014f38881895fa74d 100644
--- a/pkg/compiler/lib/src/universe/function_set.dart
+++ b/pkg/compiler/lib/src/universe/function_set.dart
@@ -295,7 +295,7 @@ class FullFunctionSetQuery implements FunctionSetQuery {
     if (_mask != null) return _mask;
     return _mask = new TypeMask.unionOf(
         functions.expand((MemberElement element) {
-          ClassEntity cls = element.enclosingClass.declaration;
+          ClassEntity cls = element.enclosingClass;
           return [cls]..addAll(closedWorld.mixinUsesOf(cls));
         }).map((cls) {
           if (closedWorld.backendClasses.nullImplementation == cls) {