DevTools failed to load SourceMap: Could not load content for http://127.0.0.1:8080/packages/build_web_compilers/src/dev_compiler_stack_trace/stack_trace_mapper.dart.js.map: HTTP error: status code 404, net::ERR_HTTP_RESPONSE_CODE_FAILURE

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This tracker is for issues related to:

* Dart core libraries ("dart:async", "dart:io", etc.)
* Dart VM
* dart2js
* dev_compiler

I followed the instructions on https://dart.dev/tutorials/web/get-started, https://dart.dev/web/debugging. And when I opened the configured chrome devtools, refresh the page. Then the console log the following warnings.

```
DevTools failed to load SourceMap: Could not load content for http://127.0.0.1:8080/packages/build_web_compilers/src/dev_compiler_stack_trace/stack_trace_mapper.dart.js.map: HTTP error: status code 404, net::ERR_HTTP_RESPONSE_CODE_FAILURE
DevTools failed to load SourceMap: Could not load content for http://127.0.0.1:8080/packages/build_web_compilers/src/dev_compiler/dart_sdk.js.map: HTTP error: status code 404, net::ERR_HTTP_RESPONSE_CODE_FAILURE
```
I can set breakpoint in dart code in source panel. The debugger stopped at the breakpoint, However the types of variable can't shown.


This is the console of `webdev serve`.
```
D:\code\dart\quickstart-web>webdev serve
[INFO] Reading cached asset graph completed, took 162ms
[INFO] Checking for updates since last build completed, took 580ms
[INFO] Serving `web` on http://127.0.0.1:8080
[INFO] Running build completed, took 205ms
[INFO] Caching finalized dependency graph completed, took 118ms
[INFO] Succeeded after 328ms with 0 outputs (0 actions)
[INFO] -------------------------------------------------------------------------------------

```

This is the chrome devtools setttings.


Reproduce the issue by following the linked Dart web tutorials, running `webdev serve`, and opening Chrome DevTools. Investigate the missing `.dart.js.map` URLs for `stack_trace_mapper.dart.js` and `dart_sdk.js`; done means the source maps load without 404 warnings and variable types are available while debugging.

Written by the indexing model from the issue text.