Update DWDS to 20.0.1 in flutter_tools (#132350)

- Updates DWDS from `19.0.2` to `20.0.1`
- Makes the required changes for the breaking changes to the `AssetReader` class

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

@@ -295,7 +295,6 @@ class WebAssetServer implements AssetReader {
         server,
         PackageUriMapper(packageConfig),
         digestProvider,
-        server.basePath,
         packageConfig.toPackageUri(
           globals.fs.file(entrypoint).absolute.uri,
         ),
@@ -347,6 +346,7 @@ class WebAssetServer implements AssetReader {
   ///
   /// It should have no leading or trailing slashes.
   @visibleForTesting
+  @override
   String basePath;
 
   // handle requests for JavaScript source, dart sources maps, or asset files.

packages/flutter_tools/pubspec.yaml

@@ -11,7 +11,7 @@ dependencies:
   args: 2.4.2
   browser_launcher: 1.1.1
   dds: 2.9.4
-  dwds: 19.0.2
+  dwds: 20.0.1
   completion: 1.0.1
   coverage: 1.6.3
   crypto: 3.0.3
@@ -107,4 +107,4 @@ dartdoc:
   # Exclude this package from the hosted API docs.
   nodoc: true
 
-# PUBSPEC CHECKSUM: 1a86
+# PUBSPEC CHECKSUM: f97d