Do not wrap the exception if we could not translate it to one of our internal exceptions.

lib/compiler/implementation/lib/js_helper.dart

Index: lib/compiler/implementation/lib/js_helper.dart
===================================================================
--- lib/compiler/implementation/lib/js_helper.dart	(revision 8097)
+++ lib/compiler/implementation/lib/js_helper.dart	(working copy)
@@ -837,9 +837,10 @@
     }
   }
 
-  // Last resort: Create a completely generic exception object and use
-  // the JS exception message as its argument.
-  return new Exception(message);
+  // Just return the exception. We should not wrap it because in case
+  // the exception comes from the DOM, it is a JavaScript
+  // object backed by a native Dart class.
+  return ex;
 }
 
 /**