If we take the flutter counter app (created via flutter create app) and modify it slightly via

dependencies:
+  helper:
+    path: ../helper

and

+ import 'package:helper/helper.dart';
...
+ int i = 0;
class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;   
                                                                                                                                        
  void _incrementCounter() {                                                                                                            
+    if (++i % 5 == 0) helper(() => throw 'baz');                                                                                                      
    setState(() {

and have the helper method in package:helper call some things which eventually call the given closure to it

// package:helper/helper.dart (aka ../helper/lib/helper.dart)
helper(void Function() fun) {
  print('helper');
  helper1(fun);
}

helper1(void Function() fun) {
  print('helper2');
  helper2(fun);
}

helper2(void Function() fun) {
  print('helper2');
  fun();
}

, then we can break on the exception in chrome.

What we'll observe is that

• The core library methods have no source maps (e.g. js_helper.dart:throwExpression)
• The lib/main.dart has valid source maps
• The package:helper/helper.dart has invalid source maps (for me it shows a html file in chrome)
• The package:flutter/* code has valid source maps

So either

• we have to instruct dart2js to emit different source maps for packages (e.g. package:helper in this case) or
• we have to make the flutter run tool's http server serve the files for the filepaths currently emitted by dart2js

(On dart2wasm this issue doesn't exist because we currently bake in absolute file url which work very well on developer machines, see dart-lang/sdk#56232 for follow up tasks)

/cc @yjbanov @sigmundch @a-siva for finding someone to own this