Trying to set breakpoints in reference-backed source files throws #1566
Description
This happens because we assume source.path is set in a breakpoint request.
rejected promise not handled within 1 second: TypeError: Cannot read property 'replace' of undefined
stack trace: TypeError: Cannot read property 'replace' of undefined
at Object.formatPathForVm (/Users/dantup/Dev/Dart-Code/out/src/debug/utils.js:75:17)
at DartDebugSession.<anonymous> (/Users/dantup/Dev/Dart-Code/out/src/debug/dart_debug_impl.js:431:27)
at Generator.next (<anonymous>)
at __awaiter (/Users/dantup/Dev/Dart-Code/out/src/debug/dart_debug_impl.js:7:71)
at new Promise (<anonymous>)
at __awaiter (/Users/dantup/Dev/Dart-Code/out/src/debug/dart_debug_impl.js:3:12)
at DartDebugSession.setBreakPointsRequest (/Users/dantup/Dev/Dart-Code/out/src/debug/dart_debug_impl.js:421:16)