Revert "+= won't let compile optimize String concats"

This reverts commit 4dc78c10eeda86e8fef4bd978fab88c2cfa31fcd.
Add an HTML5 encoder mode for HttpPostRequestEncoder
Use package-local fields instead of reflection / Reapply 4dc78c10eeda86e8fef4bd978fab88c2cfa31fcd / Inspector warnings