add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/polymer/lib/src/loader.dart

Index: pkg/polymer/lib/src/loader.dart
diff --git a/pkg/polymer/lib/src/loader.dart b/pkg/polymer/lib/src/loader.dart
index 844b6264f645a52dc9003181a727d25b0d6876c5..1011b5a2b0d2c247f8d5574c31e3bfd0e3e0b2d5 100644
--- a/pkg/polymer/lib/src/loader.dart
+++ b/pkg/polymer/lib/src/loader.dart
@@ -90,7 +90,7 @@ void _loadLibrary(String uriString, [String srcUrl]) {
     for (var m in c.metadata) {
       var meta = m.reflectee;
       if (meta is CustomTag) {
-        Polymer.register(meta.tagName, _getReflectedTypeWorkaround(c));
+        Polymer.register(meta.tagName, getReflectedTypeWorkaround(c));
       }
     }
 
@@ -103,26 +103,6 @@ void _loadLibrary(String uriString, [String srcUrl]) {
   }
 }
 
-// Horrible hack to work around: http://dartbug.com/12607
-Type _getReflectedTypeWorkaround(ClassMirror cls) {
-  // On Dart VM, just return reflectedType.
-  if (1.0 is! int) return cls.reflectedType;
-
-  var mangledName = reflect(cls).getField(_mangledNameField).reflectee;
-  Type type = _jsHelper.invoke(#createRuntimeType, [mangledName]).reflectee;
-  return type;
-}
-
-final LibraryMirror _jsHelper =
-    currentMirrorSystem().libraries[Uri.parse('dart:_js_helper')];
-
-final Symbol _mangledNameField = () {
-  var jsClassMirrorMirror = reflect(reflectClass(ClassMirror)).type;
-  for (var name in jsClassMirrorMirror.variables.keys) {
-    if (MirrorSystem.getName(name) == '_mangledName') return name;
-  }
-}();
-
 void _maybeInvoke(ObjectMirror obj, MethodMirror method) {
   var annotationFound = false;
   for (var meta in method.metadata) {