Remove use of deprecated mirror functions from Polymer.

pkg/polymer/lib/src/instance.dart

Index: pkg/polymer/lib/src/instance.dart
diff --git a/pkg/polymer/lib/src/instance.dart b/pkg/polymer/lib/src/instance.dart
index 671d9e0080f776778bd225c90b1f9b387a93f582..47809ee25abebf047826a316391dd7bf816e214b 100644
--- a/pkg/polymer/lib/src/instance.dart
+++ b/pkg/polymer/lib/src/instance.dart
@@ -794,8 +794,8 @@ abstract class Polymer implements Element, Observable, NodeBindExtension {
     // explicitly. Unless VM mirrors are optimized first, this will be expensive
     // once custom elements extend directly from Element (see issue 11108).
     var receiverMirror = reflect(receiver);
-    var method = receiverMirror.type.methods[methodName];
-    if (method != null) {
+    var method = receiverMirror.type.declarations[methodName];
+    if (method != null && method is MethodMirror) {
       // This will either truncate the argument list or extend it with extra
       // null arguments, so it will match the signature.
       // TODO(sigmund): consider accepting optional arguments when we can tell