Fix dart2js compile warning
Dynamic aceContainer instead of static one in HTML: fixes #3676

* The static div#aceContainer was out-of-place in the HTML: AceManager would rip it out of the DOM and reinsert in a different place when opening an editor. At the same time, AceManager would create an AceEditor object on startup even when there were no tabs opened. These two facts resulted in a misplaced div#aceContainer with an empty editor protruding from under the rest of the UI in form of the weird line from #3676.
* Also rearranged the code in AceManager (a result of an unused experimentation, but useful still).
Adding test for no files found
Merge branch 'master' of https://github.com/dart-lang/chromedeveditor into umop-preferences-editorconfig-parse-simple
Merge remote-tracking branch 'origin/dynamic-ace-container-element' into consolidate-ui-in-topUi
Merge branch 'small-fixes-4' into consolidate-ui-in-topUi
Consolidate all UI in spark-polymer-ui (except for <template>s and little else)
Address comments
Merge pull request #3753 from ussuri/dynamic-ace-container-element

Dynamic aceContainer instead of static one in HTML: fixes #3676
Merge pull request #3755 from ussuri/small-fixes-4

Set Roboto in widgets conservatively + final -> const for 1 variable
Merge remote-tracking branch 'upstream/master' into consolidate-ui-in-topUi
Fix bug introduced in prev commit
Merge pull request #3756 from ussuri/dynamic-ace-container-element

Fix bug introduced in prev commit
Branch 0.18
Remove no-longer-relevant build script

I was the only user of it ever, and I've settled on an alternative build sequence.
Merge pull request #3757 from ussuri/minor-cleanups

Remove no-longer-relevant build script
Pin Polymer version in templates to `latest`

...now that the GitHub access rate limit is no longer an issue, which was the reason for the original back-and-forth between master-latest-master (master doesn't go through version resolution, so didn't hit the limit). The rate limit was fixed in #3733.
Merge pull request #3758 from ussuri/pin-polymer-in-templates-2

Pin Polymer version in templates to `latest`
Update release notes for 0.18
Remove support for custom JS dependencies in project type spec

This form of project type specification in spark_polymer_ui.html is no longer used, because it used to put explicit knowledge about Polymer into spark.dart: "js-web-app-polymer;polymer,paper-elements".

Conversely, since the instantiation code already had some support for multi-template projects, fully implemented it (it was very easy and quick), so the "template-a+template-b+templace-c" form is now supported.