Fix mirror_loader to detect whether the type is a generic class.

pkg/polymer/lib/src/mirror_loader.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.
 
 /// Contains logic to initialize polymer apps during development. This
 /// implementation uses dart:mirrors to load each library as they are discovered
 /// through HTML imports. This is only meant to be during development in
 /// dartium, and the polymer transformers replace this implementation with
 /// codege generation in the polymer-build steps.
 library polymer.src.mirror_loader;
@@ skipping 144 matching lines @@
     //    in dart2js, so they end up being called twice)
     //  - dartbug.com/12134 (sometimes "method.metadata" throws an exception,
     //    we could wrap and hide those exceptions, but it's not ideal).
   }
 
   initializers.addAll(customTags.values);
 }
 
 void _loadCustomTags(LibraryMirror lib, ClassMirror cls,
     LinkedHashMap registerFns) {
-  if (cls == null || cls.reflectedType == HtmlElement) return;
+  if (cls == null) return;
+  if (cls.hasReflectedType && cls.reflectedType == HtmlElement) return;
 
   // Register superclass first.
   _loadCustomTags(lib, cls.superclass, registerFns);
 
   if (cls.owner != lib) {
     // Don't register classes from different libraries.
     // TODO(jmesserly): @CustomTag does not currently respect re-export, because
     // LibraryMirror.declarations doesn't include these.
     return;
   }
 
   var meta = _getCustomTagMetadata(cls);
   if (meta == null) return;
 
+  if (!cls.hasReflectedType) {
+    var name = MirrorSystem.getName(cls.simpleName);
+    new Completer().completeError(new UnsupportedError('Custom element classes '
+        'cannot have type-parameters: $name'));
+    return;
+  }
+
   registerFns.putIfAbsent(cls.reflectedType, () =>
       () => Polymer.register(meta.tagName, cls.reflectedType));
 }
 
 /// Search for @CustomTag on a classemirror
 CustomTag _getCustomTagMetadata(ClassMirror c) {
   for (var m in c.metadata) {
     var meta = m.reflectee;
     if (meta is CustomTag) return meta;
   }
@@ skipping 15 matching lines @@
         " ${method.simpleName} is not.");
     return;
   }
   if (!method.parameters.where((p) => !p.isOptional).isEmpty) {
     print("warning: methods marked with @initMethod should take no "
         "arguments, ${method.simpleName} expects some.");
     return;
   }
   initializers.add(() => obj.invoke(method.simpleName, const []));
 }