First cut at behaviors. This just implements the lifecycle methodsportion. We may get the rest for …

test/src/common/behavior_test.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+@TestOn('browser')
+library polymer.test.src.common.js_proxy_test;
+
+import 'dart:html';
+import 'dart:js';
+import 'package:polymer/polymer.dart';
+import 'package:test/test.dart';
+import 'package:web_components/web_components.dart';
+
+main() async {
+  await initPolymer();
+  MyElement el;
+
+  group('lifecycle methods', () {
+    setUp(() {
+      el = new MyElement();
+    });
+
+    test('created', () {
+      expect(el.dartInvocations['created'], [
+        [el]
+      ]);
+      expect(el.jsElement['jsInvocations']['created'], [
+        [el]
+      ]);
+      expect(el.dartInvocations['attached'], isEmpty);
+      expect(el.dartInvocations['detached'], isEmpty);
+      expect(el.dartInvocations['attributeChanged'], isEmpty);
+    });
+
+    test('attached', () {
+      document.body.append(el);
+      expect(el.dartInvocations['attached'], [
+        [el]
+      ]);
+      expect(el.jsElement['jsInvocations']['attached'], [
+        [el]
+      ]);
+      expect(el.dartInvocations['detached'], isEmpty);
+      expect(el.dartInvocations['attributeChanged'], isEmpty);
+    });
+
+    test('detached', () {
+      document.body.append(el);
+      el.remove();
+      expect(el.dartInvocations['detached'], [
+        [el]
+      ]);
+      expect(el.jsElement['jsInvocations']['detached'], [
+        [el]
+      ]);
+      expect(el.dartInvocations['attributeChanged'], isEmpty);
+    });
+
+    test('attributeChanged', () {
+      el.attributes['foo'] = 'bar';
+      expect(el.dartInvocations['attributeChanged'], [
+        [el, 'foo', null, 'bar']
+      ]);
+      expect(el.jsElement['jsInvocations']['attributeChanged'], [
+        [el, 'foo', null, 'bar']
+      ]);
+    });
+  });
+}
+
+@BehaviorProxy(const ['JsBehavior'])
+abstract class JsBehavior implements CustomElementProxyMixin {
+  JsArray get jsInvocations => jsElement['jsInvocations'];
+}
+
+@Behavior()
+class DartBehavior {
+  Map<String, List<List>> dartInvocations = {
+    'created': [],
+    'attached': [],
+    'detached': [],
+    'attributeChanged': []
+  };
+
+  static created(DartBehavior thisArg) {
+    thisArg.dartInvocations['created'].add([thisArg]);
+  }
+
+  static attached(DartBehavior thisArg) {
+    thisArg.dartInvocations['attached'].add([thisArg]);
+  }
+
+  static detached(DartBehavior thisArg) {
+    thisArg.dartInvocations['detached'].add([thisArg]);
+  }
+
+  static attributeChanged(DartBehavior thisArg, String name, type, value) {
+    thisArg.dartInvocations['attributeChanged']
+        .add([thisArg, name, type, value]);
+  }
+}
+
+@jsProxyReflectable
+@PolymerRegister('my-element')
+class MyElement extends PolymerElement with JsBehavior, DartBehavior {
+  MyElement.created() : super.created();
+
+  factory MyElement() => document.createElement('my-element');
+}