Replace bootstrap logic with 'boot.js', use 'component/dart' mime-type and add

pkg/polymer/test/mirror_loader_3.dart

Index: pkg/polymer/test/mirror_loader_3.dart
diff --git a/pkg/polymer/test/mirror_loader_3.dart b/pkg/polymer/test/mirror_loader_3.dart
new file mode 100644
index 0000000000000000000000000000000000000000..1f3e511b73f69131fb61f693b3e3b05c2d489c6c
--- /dev/null
+++ b/pkg/polymer/test/mirror_loader_3.dart
@@ -0,0 +1,11 @@
+library mirror_loader_test3;
+
+import 'dart:html';
+
+import 'package:polymer/polymer.dart';
+
+@CustomTag('x-b')
+class XB<T> extends PolymerElement {
+  final String x = "a";
+  XB.created() : super.created();
+}