Serve web source maps in release runner (#45318)

packages/flutter_tools/lib/src/build_runner/web_fs.dart

@@ -353,11 +353,27 @@ abstract class AssetServer {
 }
 
 class ReleaseAssetServer extends AssetServer {
+  // Locations where source files, assets, or source maps may be located.
+  final List<Uri> _searchPaths = <Uri>[
+    fs.directory(getWebBuildDirectory()).uri,
+    fs.directory(Cache.flutterRoot).parent.uri,
+    fs.currentDirectory.childDirectory('lib').uri,
+  ];
+
   @override
   Future<Response> handle(Request request) async {
-    final Uri artifactUri = fs.directory(getWebBuildDirectory()).uri.resolveUri(request.url);
-    final File file = fs.file(artifactUri);
-    if (file.existsSync()) {
+    Uri fileUri;
+    for (Uri uri in _searchPaths) {
+      final Uri potential = uri.resolve(request.url.path);
+      if (potential == null || !fs.isFileSync(potential.toFilePath())) {
+        continue;
+      }
+      fileUri = potential;
+      break;
+    }
+
+    if (fileUri != null) {
+      final File file = fs.file(fileUri);
       final Uint8List bytes = file.readAsBytesSync();
       // Fallback to "application/octet-stream" on null which
       // makes no claims as to the structure of the data.

packages/flutter_tools/test/general.shard/web/asset_server_test.dart

@@ -4,6 +4,7 @@
 
 import 'package:flutter_tools/src/artifacts.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
+import 'package:flutter_tools/src/base/io.dart';
 import 'package:flutter_tools/src/build_runner/web_fs.dart';
 import 'package:flutter_tools/src/globals.dart';
 import 'package:flutter_tools/src/project.dart';
@@ -151,4 +152,26 @@ void main() {
       'content-length': '28',
     });
   }));
+
+  test('release asset server serves content from flutter root', () => testbed.run(() async {
+    assetServer = ReleaseAssetServer();
+    fs.file(fs.path.join('flutter', 'bar.dart'))
+      ..createSync(recursive: true)
+      ..writeAsStringSync('void main() { }');
+    final Response response = await assetServer
+      .handle(Request('GET', Uri.parse('http://localhost:8080/flutter/bar.dart')));
+
+    expect(response.statusCode, HttpStatus.ok);
+  }));
+
+  test('release asset server serves content from project directory', () => testbed.run(() async {
+    assetServer = ReleaseAssetServer();
+    fs.file(fs.path.join('lib', 'bar.dart'))
+      ..createSync(recursive: true)
+      ..writeAsStringSync('void main() { }');
+    final Response response = await assetServer
+      .handle(Request('GET', Uri.parse('http://localhost:8080/bar.dart')));
+
+    expect(response.statusCode, HttpStatus.ok);
+  }));
 }