change to use a hosted dep on polymer_elements

@ussuri @terrylucas 

This converts our polymer_elements dependency from git to hosted. It looks like `polymer_ui_elements` has been changed to depend on it in a hosted manner, we depend on it using a git dep, and pub doesn't like the two different types of deps used. I don't see direct usage of polymer_ui_elements, but perhaps polymer_elements depends on it?

    [~/projects/spark/ide] pub install
    Resolving dependencies..........
    Incompatible dependencies on 'polymer_elements':
     - 'polymer_ui_elements' depends on it from source hosted
     - 'spark' depends on it from source git

I tried removing the reference entirely from the pubspec files, but it looks like the polymer_elements library is still used by the polymer version. This CL does fix the build, but the polymer version of the app no longer starts up. @ussuri if you want to investigate a better fix, sgtm.