tutorials/httpserver: update to match code changes
Updating barback examples. Also updating advice on where to put assets.

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R=kathyw@google.com, rnystrom@google.com

Review URL: https://codereview.appspot.com/85870043
Clarify version on polymer.dart page

https://polymer-dot-dart-lang.appspot.com/polymer-dart/

Moved the pubspec.yaml section up, changed version from "any" to a range, and added a note about the version.

Also fixed dartbug.com/16246.

R=jmesserly@google.com

Review URL: https://chromiumcodereview.appspot.com/238363002
Merge pull request #327 from chalin/patch-0410-httpserver

tutorials/httpserver: update to match code changes and other minor edits
Fixing a reader-reported error and updating the screenshot.

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R=gbracha@google.com

Review URL: https://codereview.appspot.com/88040047
Clarifying where to put transformer code.

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R=kathyw@google.com, nweiz@google.com

Review URL: https://codereview.appspot.com/87810043
Adding Java info and a link to the troubleshooting page.

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Review URL: https://codereview.appspot.com/88990043
Adding info on pub cache repair.

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R=kathyw@google.com

Review URL: https://codereview.appspot.com/88010046