Reduce usage of .enclosingElement to get enclosing class.

lib/compiler/implementation/native_handler.dart

Index: lib/compiler/implementation/native_handler.dart
diff --git a/lib/compiler/implementation/native_handler.dart b/lib/compiler/implementation/native_handler.dart
index 8592b03be9d7b0cb08543242e23b7caa2ba2229d..9240c555e22a32b454cf65371200e34e3860926c 100644
--- a/lib/compiler/implementation/native_handler.dart
+++ b/lib/compiler/implementation/native_handler.dart
@@ -188,7 +188,7 @@ void handleSsaNative(SsaBuilder builder, Expression nativeBody) {
   // not a JS property on the DOM types, returns the type name.
   if (element.name == const SourceString('typeName')
       && element.isGetter()
-      && nativeEmitter.toNativeName(element.enclosingElement) == 'DOMType') {
+      && nativeEmitter.toNativeName(element.getEnclosingClass()) == 'DOMType') {
     Element methodElement =
         compiler.findHelper(const SourceString('getTypeNameOf'));
     HStatic method = new HStatic(methodElement);