Introduce frontend_server that flutter tools will use for compilation. (#3982)

* Introduce frontend_server that allows for interactive and batch modes.

Example of batch mode:
```
$ dart $HOME/p/f/t11/flutter/engine/src/flutter/frontend_server/bin/server.dart --sdk-root=$HOME/p/f/t11/flutter/engine/src/out/android_debug/flutter_patched_sdk --platform-kernel-dill=$HOME/p/f/t11/flutter/engine/src/out/android_debug/flutter_patched_sdk/platform.dill $HOME/p/f/t11/flutter/flutter/examples/flutter_gallery/lib/main.dart

out: result 79256789-598c-4909-a9e4-0ef36d4c26ba
out: 'State&TickerProviderStateMixin^#U0^' is exported from both 'package:flutter/src/widgets/animated_cross_fade.dart' and 'package:flutter/src/widgets/animated_list.dart'.
...
out: Superclass has no method named 'hitTest'.
out: 79256789-598c-4909-a9e4-0ef36d4c26ba $HOME/p/f/t11/flutter/flutter/examples/flutter_gallery/lib/main.dart.dill
$
```

Example of interactive mode - communication via stdin/stdout:
```
$ dart $HOME/p/f/t11/flutter/engine/src/flutter/frontend_server/bin/server.dart --sdk-root=$HOME/p/f/t11/flutter/engine/src/out/android_debug/flutter_patched_sdk --platform-kernel-dill=$HOME/p/f/t11/flutter/engine/src/out/android_debug/flutter_patched_sdk/platform.dill --incremental

out: Frontend server is ready.
in: compile abc.dart
out: result fd99c0f9-12bb-4083-818e-2521d454fdc8
out: fd99c0f9-12bb-4083-818e-2521d454fdc8 abc.dart.dill
in: accept
in: recompile kuka
in: 123.dart
in: 456.dart
in: kuka
out: result d12f8d05-6c79-44f7-a775-1d3469e1be53
in: quit
$
```

* Add test. Cleanup dependencies.

* One more test with mocks

* Remove platform-kernel option. Refactored CompilerOptions. Clean up comments.

* Update license

* Use Uri.base.resolve consistently. Ensure folder path for sdk root. Fix test.

* Fix main_dart for frontend_server build target

* Reintroduce .packages file, add README.md, revert changes to licenses_flutter

* Run analyzer on frontend-server. Update .packages deps so tests can be run too.

* Add analyzer options file

* Clean up dependencies

* Add frontend_server files to license file back

* Fix comment in analysis_options.yaml

* Run frontend_server tests in travis

* Update licenses

* Change a+x permission on travis/test.sh

* pub get before pub run test

* Fix path to when

* Clarify accept/reject instructions

* Formatting, fixed docs, style

* Update license script to skip .travis.yml, when/example. Update license file.

* Update style nits

* Fix license

* Trim license extracted text. Fix licenses

* Update licenses_third_party signature

* Few more tests

* Update dart license

* Update dart license

* Style changes. Fix return value for Future<Null>