in-line the debugger code from the Dart plugin

[devoncarew]

This allow us to iterate much more quickly, w/o having to wait for releases of the Dart plugin to the stable channel. This might look something like:

• assume that the flutter-intellij and intellij-plugins repos are located in the same parent folder
• write a script to sync the debugger code from intellij-plugins/Dart into flutter-intellij/third_party; change the package prefix at the same time (to avoid accidently loading the Dart plugins classes from its classloader)
• have the script be able to sync the other way as well. We can then periodically sync updates from the flutter plugin to the Dart one, and vice versa
• update the flutter debugger to use the new classes

[devoncarew]

I'm going to start work on a script to sync the debugger libraries.