Add DartDevcBootstrapTransformer and compiler tests for dartdevc (#1582)

Some additional updates based on testing with PPW (now that we have an e2e to play with):

* add sourcemaps to output and update tests
* use strong mode summaries, support filenames with dots, add a fake sourcemap to satisfy package test
* add better error if the module reader fails to find a module for a file
* stop diet parsing, remove --unsafe-force-compile
* add environment constant support to the DartDevcModuleTransformer
* don't output ddc resources in folders with no entrypoints
* don't create sourcemaps in release mode
* update relevant compiler tests to run with dartdevc