Remove use of deprecated mirror API from code that only runs against dart2js.

pkg/polymer/lib/src/reflected_type.dart

Index: pkg/polymer/lib/src/reflected_type.dart
diff --git a/pkg/polymer/lib/src/reflected_type.dart b/pkg/polymer/lib/src/reflected_type.dart
index ad11ffb61f0af33c504f97c7431c71d3470e6c1a..6486abc4659a396f40e2c07c8875b920844df430 100644
--- a/pkg/polymer/lib/src/reflected_type.dart
+++ b/pkg/polymer/lib/src/reflected_type.dart
@@ -25,7 +25,7 @@ final LibraryMirror _jsHelper =
 
 final Symbol _mangledNameField = () {
   var jsClassMirrorMirror = reflect(reflectClass(ClassMirror)).type;
-  for (var name in jsClassMirrorMirror.variables.keys) {
+  for (var name in jsClassMirrorMirror.declarations.keys) {
     if (MirrorSystem.getName(name) == '_mangledName') return name;
   }
 }();