Merge branch 'master' into canary

Re-bundle karma test files when non-test JS files change.

Not sure there is a better way to do this, but apparently karma only watches the test files themselves which is not really what one wants during test driven development or really ever.

Fixes ampproject#1623

Merge remote-tracking branch 'origin/master' into canary

Fix regression where polyfills where build into into extensions.

Merge remote-tracking branch 'origin/master' into canary

use --version flag value for release label and tag

Merge remote-tracking branch 'origin/master' into canary

Merge remote-tracking branch 'origin/master' into canary

Merge remote-tracking branch 'origin/master' into canary

Merge remote-tracking branch 'origin/master' into canary