Concentrate knowledge/handling of what Pub can/can't support in Pub itself

* Move throwing "Pub can't run on Windows" from PackageManagementAction to Pub, where another similar exception is already thrown.
* Also, fix an accidentally discovered bug in PackageServiceProperties.isSecondaryPackageResource().
Merge pull request #3817 from ussuri/small-fixes-4

Polymer-related: platform.js -> webcomponents.min.js
Merge pull request #3816 from ussuri/pub-fixes

Concentrate knowledge/handling of what Pub can/can't support in Pub itself
Fix a few compile-time warnings