Added linter backend and lint output for a few scenarios

     * Added a backend for linter
     * Added log events for TreeBuilderFixups
     * Added log events for Fostered content
     * Added log events for mixed templates

Change-Id: Id7a94a0f4f025981fa53bb13c4c01291e14e37a1
Merge "Added linter backend and lint output for a few scenarios"