Support for specifying an output directory (issue #106)

lib/web_component.dart

Index: lib/web_component.dart
diff --git a/lib/web_component.dart b/lib/web_component.dart
index 57fe37374c68c942d35809660cce2af24b113f57..31ae9b39dc3b32f77c3b749a4dc630c860b38363 100644
--- a/lib/web_component.dart
+++ b/lib/web_component.dart
@@ -29,7 +29,7 @@ abstract class WebComponent implements Element {
    * Use [WebComponent.forElement] instead.
    */
   WebComponent() : _element = null {
-    throw new UnsupportedOperationException(
+    throw new UnsupportedError(
         'Directly constructing web components is not currently supported. '
         'You need to use the WebComponent.forElement constructor to associate '
         'a component with its DOM element. If you run "bin/dwc.dart" on your '