Port b-secret to Polymer Dart

- removed CustomEvents
- added data binding for value


### Known issue:
"required" wouldn't work:  if "required" is set on an input element it is always required:
e.g.  `<input required="false">` is required, `<input required>` is required
`<input>` is not required
what does the builder do and what does the pubspec do actually?

eg i just added the paths/components to both files and then things worked
but iirc it sometimes also worked if i did not put them there (? not sure about this anymore)

also just curious if there is a 'manual' way to do the same thing that the dart editor is doing?
eg something like 'pub build .' or 'pub serve .' ?