Merge pull request #1520 from ussuri/bower-support-transitive-deps-2

Fix typo in bower.dart that prevented Bower from downloading anything
Refactor: moved Bower package fetching code from BowerManager to _BowerFetcher
Re-add transitive Bower dependencies v.1.0 before proceding to v.2.0
Fix a typo that broke Pub support (and tests)
Merge pull request #1522 from ussuri/bower-support-transitive-deps

Fix a typo that broke Pub support (and tests)
Merge remote-tracking branch 'upstream/master' into bower-support-transitive-deps-2
Transitive Bower dependency downloading v.2.0

Downloads in 2 stages:
1) Recursively XHRs all bower.json's starting at the project's top bower.json. Record dependencies for every one.
2) Downloads the resulting set of dependencies in parallel.