Fix some users of removed mirror API.

pkg/polymer/lib/src/declaration.dart

Index: pkg/polymer/lib/src/declaration.dart
diff --git a/pkg/polymer/lib/src/declaration.dart b/pkg/polymer/lib/src/declaration.dart
index 84dbc1ae5380a29438013f6638a2ed5ae0c09f0d..030333bb4ebb3abfcce330a5554d1602de1be872 100644
--- a/pkg/polymer/lib/src/declaration.dart
+++ b/pkg/polymer/lib/src/declaration.dart
@@ -222,8 +222,10 @@ class PolymerDeclaration extends HtmlElement {
     var cls = reflectClass(type);
     // TODO(jmesserly): this feels unnatrual in Dart. Since we have convenient
     // lazy static initialization, can we get by without it?
-    var registered = cls.methods[#registerCallback];
-    if (registered != null && registered.isStatic &&
+    var registered = cls.declarations[#registerCallback];
+    if (registered != null &&
+        registered is MethodMirror &&
+        registered.isStatic &&
         registered.isRegularMethod) {
       cls.invoke(#registerCallback, [this]);
     }