reloadSources should wait until new sources are parsed before finishing/computing location information

This explains some of the flakiness on the Windows bots when running hot_reload_breakpoints_test.dart in this PR: #2627. I was able to repro using my local Windows machine, but couldn't repro this on a Mac.

Specifically, when adding a breakpoint after a hot reload, we sometimes refer to the old JS script ID still instead of the new one because we didn't wait until we parsed the new scripts before fetching the JS script ID. _chromePathToRuntimeScriptId is the map in question:

webdev/dwds/lib/src/debugging/debugger.dart

Line 491 in 7f376d2

chromePathToRuntimeScriptId[scriptPath] = e.script.scriptId;

. Used when adding breakpoints here:

webdev/dwds/lib/src/debugging/modules.dart

Line 72 in 7f376d2

return chromePathToRuntimeScriptId[serverPath];

.

I'll likely address this at the same time as #2628.