Small fix to boot.js so it can work with CSP if an existing html-import tag is present.
Small fix to boot.js so it can work with CSP if an existing html-import tag is
present.

Note: this will be deleted soon when we deprecate boot.js (a week or so).

R=jmesserly@google.com, sergeygs@chromium.org

Review URL: https://codereview.chromium.org//27246006

git-svn-id: 0aa7e0b98478d6ef7145d04e8154f8ab2209deba@28675 260f80e4-7a28-3924-810f-c04153c831b5

meflyup/polymer-dart,dam0vm3nt/polymer-dart,ronlobo/polymer-dart,jakemac53/polymer-dart,ronlobo/polymer-dart,meflyup/polymer-dart,dart-archive/polymer-dart,dam0vm3nt/polymer-dart,jakemac53/polymer-dart,dart-archive/polymer-dart