setBreakpoints is called with sourceModified:false when source is dirty and new breakpoint is added

It's not clear from the DAP spec exactly what the intended behaviour is here, so I've filed https://github.com/microsoft/debug-adapter-protocol/issues/376. However, this part definitely seems like a bug. I added a breakpoint to a file, modified the file, and then added another. At no point did I save the file.  In the DAP log, I see that VS Code called `setBreakpoints` after I added the second breakpoint but with `sourceModified: false`. This seems to be incorrect.. The debug adapter now thinks these breakpoints are for a saved file, but they are not. ``` [11:08:17] [DAP] [Info] ==> {"command":"setBreakpoints","arguments":{"source":{"name":"package:flutter_counter/main.dart","path":"/Users/danny/Desktop/dart_samples/flutter_counter/lib/main.dart","sourceReference":0},"lines":[14],"breakpoints":[{"line":14}],"sourceModified":false},"type":"request","seq":8} [11:08:21] [DAP] [Info] ==> {"command":"setBreakpoints","arguments":{"source":{"name":"package:flutter_counter/main.dart","path":"/Users/danny/Desktop/dart_samples/flutter_counter/lib/main.dart","sourceReference":0},"lines":[14,15],"breakpoints":[{"line":14},{"line":15}],"sourceModified":false},"type":"request","seq":9} ```