Debugging should handle identical sources in sourceReference better

I'm trying to improve the debugging experience for Flutter when using Hot Reload. There are a few places where the behaviour is currently confusing: 1. The user modifies a file in the editor and then hits a breakpoint - the file contents on screen do not match what the VM is running 2. The user modifies a file *and saves*, which results in new breakpoints being sent to the VM with the new locations - however if the VM was not hot reloaded, we just moved all the breakpoints in incorrect locations 3. Even after a hot reload, it's possible for the VM to pause in a location based on old source code (currently it will take the user to the new source code in the editor, which is not what's actually being executed) I thought many of these issues could be resolved by using `stackTraceRequest` in the `getStackTrace` call to give the real source code back to VS Code. This works reasonable well when the source codes are *different* (eg. the user modified a file and saved, but we did NOT hot reload). They end up getting a new editor window showing the source code we're executing. However, if the source files are identical, the debug experience is poor. VS Code still opens a new editor window (even though the contents are the same as what they already had open) and: 1. This editor is read-only. This prevents the user from making changes to then hot-reload 2. VS Code treats this as a completely different file (even though the paths are the same) and therefore it does not show any of the breakpoints that were set, and adding new breakpoints will result in a "different list" being sent to the debug adapter which is very confusing. For example here, these two files are the same - the one on the left is the "real" file and the one on the right is the `sourceReference`-provided file (but with the same `path`). Notice they have different breakpoints (which cannot be distinguished in the Breakpoints list): ![Screenshot 2019-11-21 at 10 40 10 am](https://user-images.githubusercontent.com…