To support Flutter to be a branch based build, test and release process, both cocoon backend and frontend need to be updated.

Backend will support branch property in commit model, and also support branch matching to pre-defined regular expressions.

Following backend changes

• basic support (Backend support branching for flutter repo cocoon#658)
• list commits based on branches, and update indexes for checklists to use the the branch (Support branch parameter for API get-status cocoon#668)
• Use both branch and SHA for commit Key (Add branch to commit key cocoon#684)
• create /api/branches to return available branches (Create /api/get-branches to support frontend listing branches cocoon#682)
• calculate tree status based on branches (Support branch for tree build status cocoon#679)
• calculate cirrus task status based on branches (Support branching for cirrus status cocoon#688)
• update reserve-task API to pick tasks regardless of branch (Reserve task regardless of branches cocoon#695)
• push build status to github for all branches (Support branching for push build status to github cocoon#702)
• push gold status to github for all branches (requires support from Gold for branching)
• refresh chromebot status for all branches (Support branching for refresh chromebot status cocoon#720)

Following frontend changes - Build page

• query branches from backend (Cocoon getFlutterBranches() cocoon#686)
• update build state to track branches (Explicitly use the master branch in build dashboard cocoon#693)
• pass build state branches to cocoon server for fetching commit statuses (Explicitly use the master branch in build dashboard cocoon#693)
• update branches method in flutter build state to change branch (Branch dropdown menu on build dashboard cocoon#700)
• add dropdown to build dashboard to interacting with the branches on build state (Branch dropdown menu on build dashboard cocoon#700)

Following frontend changes - performance page

• update Branch property in entity TimeseriesValue
• add branch support on dashboard (Add branching support for performance dashboard cocoon#751)
• add branch when querying historical data (Add branch filter when querying TimeseriesValue for benchmark history cocoon#752)
• append prior master data to release branch for benchmark dashboard (Append old master data to release branch for /api/public/get-benchmarks cocoon#753)