Use baseUri always when doing a hot reload or hot restart (flutter#172271)

We already use the baseUri when computing hot reload sources metadata as
it can never be null. The member is changed to be non-nullable to
reflect that.

To be consistent, we also use the baseUri (full url) for a hot restart
when running with the DDC library bundle format.

Related PR: dart-lang/webdev#2650

## 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].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] 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.

Author: srujzs

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

@@ -202,7 +202,7 @@ class WebDevFS implements DevFS {
   Set<String> get assetPathsToEvict => const <String>{};
 
   @override
-  Uri? get baseUri => webAssetServer.baseUri;
+  Uri get baseUri => webAssetServer.baseUri;
 
   @override
   Future<Uri> create() async {
@@ -234,7 +234,7 @@ class WebDevFS implements DevFS {
       platform: platform,
       shouldEnableMiddleware: shouldEnableMiddleware,
     );
-    return baseUri!;
+    return baseUri;
   }
 
   @override

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

@@ -122,7 +122,7 @@ class WebAssetServer implements AssetReader {
     }
     if (writeRestartScripts) {
       final srcIdsList = <Map<String, String>>[
-        for (final String src in modules) <String, String>{'src': src, 'id': src},
+        for (final String src in modules) <String, String>{'src': '$baseUri/$src', 'id': src},
       ];
       writeFile('restart_scripts.json', json.encode(srcIdsList));
     }
@@ -161,7 +161,7 @@ class WebAssetServer implements AssetReader {
             as Map<String, dynamic>,
       );
       final List<String> libraries = metadata.libraries.keys.toList();
-      final moduleUri = baseUri != null ? '$baseUri/$module' : module;
+      final moduleUri = '$baseUri/$module';
       moduleToLibrary.add(<String, Object>{
         'src': moduleUri,
         'module': metadata.name,
@@ -176,8 +176,8 @@ class WebAssetServer implements AssetReader {
     return _webMemoryFS.write(codeFile, manifestFile, sourcemapFile, metadataFile);
   }
 
-  Uri? get baseUri => _baseUri;
-  Uri? _baseUri;
+  Uri get baseUri => _baseUri;
+  late Uri _baseUri;
 
   /// Start the web asset server on a [hostname] and [port].
   ///
@@ -351,8 +351,8 @@ class WebAssetServer implements AssetReader {
                   ),
                 ),
                 packageConfigPath: buildInfo.packageConfigPath,
-                hotReloadSourcesUri: server._baseUri!.replace(
-                  pathSegments: List<String>.from(server._baseUri!.pathSegments)
+                hotReloadSourcesUri: server._baseUri.replace(
+                  pathSegments: List<String>.from(server._baseUri.pathSegments)
                     ..add(_reloadScriptsFileName),
                 ),
               ).strategy

packages/flutter_tools/lib/src/web/bootstrap.dart

@@ -267,7 +267,7 @@ $_simpleLoaderScript
             for (var i = 0; i < scripts.length; i++) {
               var script = scripts[i];
               if (script.id == null) continue;
-              var src = _currentDirectory + script.src.toString();
+              var src = script.src.toString();
               var oldSrc = window.\$dartLoader.moduleIdToUrl.get(script.id);
 
               // We might actually load from a different uri, delete the old one