Use baseUri of WebAssetServer for reload_scripts.json (#169267)

This allows embedded Flutter Web apps, hosted on a different URL than
the host app, to hot reload the correct files.

This fixes issues mentioned in:
- https://github.com/dart-lang/sdk/issues/60776
-
https://github.com/flutter/flutter/issues/88434#issuecomment-2888413942

It specifically allows you to use hot reload when debugging an embedded
Flutter Web app, running in the spawned Chrome browser via the `-d
chrome` flag, as mentioned in
https://github.com/dart-lang/sdk/issues/60776#issuecomment-2900498970

## Result

This would result in the following `reload_scripts.json` for an embedded
Flutter web app, running on `localhost:8081`:
```json
[{
    "src": "http://localhost:8081/packages/web_hot_reload/main.dart.lib.js",
    "libraries": ["package:web_hot_reload/main.dart"]
}]
```

## Normal Flutter Web apps

This also adds the baseUri to the maps for normal Flutter Web apps, for
example:
```json
[{
    "src": "http://localhost:54609/packages/web_hot_reload/main.dart.lib.js",
    "libraries": ["package:web_hot_reload/main.dart"]
}]
```
instead of the previous:
```json
[{
    "src": "packages/web_hot_reload/main.dart.lib.js",
    "libraries": ["package:web_hot_reload/main.dart"]
}]
```

While both actually work, I'm not sure if there might be cases in which
this would have another side effect.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
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https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
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https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

packages/flutter_tools/lib/src/isolated/devfs_web.dart

@@ -191,7 +191,8 @@ class WebAssetServer implements AssetReader {
   /// contains a list of objects each with two fields:
   ///
   /// `src`: A string that corresponds to the file path containing a DDC library
-  /// bundle.
+  /// bundle. To support embedded libraries, the path should include the
+  /// `baseUri` of the web server.
   /// `libraries`: An array of strings containing the libraries that were
   /// compiled in `src`.
   ///
@@ -199,7 +200,7 @@ class WebAssetServer implements AssetReader {
   /// ```json
   /// [
   ///   {
-  ///     "src": "<file_name>",
+  ///     "src": "<baseUri>/<file_name>",
   ///     "libraries": ["<lib1>", "<lib2>"],
   ///   },
   /// ]
@@ -215,7 +216,8 @@ class WebAssetServer implements AssetReader {
             as Map<String, dynamic>,
       );
       final List<String> libraries = metadata.libraries.keys.toList();
-      moduleToLibrary.add(<String, Object>{'src': module, 'libraries': libraries});
+      final String moduleUri = baseUri != null ? '$baseUri/$module' : module;
+      moduleToLibrary.add(<String, Object>{'src': moduleUri, 'libraries': libraries});
     }
     writeFile(_reloadScriptsFileName, json.encode(moduleToLibrary));
   }