Don't crash on requests for invalid package URLs (#59250)

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

@@ -517,12 +517,15 @@ class WebAssetServer implements AssetReader {
     // The file might have been a package file which is signaled by a
     // `/packages/<package>/<path>` request.
     if (segments.first == 'packages') {
-      final File packageFile = globals.fs.file(_packages.resolve(Uri(
-        scheme: 'package', pathSegments: segments.skip(1))));
+      final Uri filePath = _packages.resolve(Uri(
+        scheme: 'package', pathSegments: segments.skip(1)));
+      if (filePath != null) {
+        final File packageFile = globals.fs.file(filePath);
         if (packageFile.existsSync()) {
           return packageFile;
         }
       }
+    }
 
     // Otherwise it must be a Dart SDK source or a Flutter Web SDK source.
     final Directory dartSdkParent = globals.fs

packages/flutter_tools/lib/src/test/flutter_web_platform.dart

@@ -41,7 +41,6 @@ import '../dart/package_map.dart';
 import '../globals.dart' as globals;
 import '../project.dart';
 import '../web/chrome.dart';
-
 import 'test_compiler.dart';
 import 'test_config.dart';
 
@@ -222,6 +221,7 @@ class FlutterWebPlatform extends PlatformPlugin {
         scheme: 'package',
         pathSegments: request.requestedUri.pathSegments.skip(1),
       ));
+      if (fileUri != null) {
         final String dirname = p.dirname(fileUri.toFilePath());
         final String basename = p.basename(fileUri.toFilePath());
         final shelf.Handler handler = createStaticHandler(dirname);
@@ -237,6 +237,7 @@ class FlutterWebPlatform extends PlatformPlugin {
         );
         return handler(modifiedRequest);
       }
+    }
     return shelf.Response.notFound('Not Found');
   }
 

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

@@ -364,6 +364,13 @@ void main() {
     expect(response.statusCode, HttpStatus.notFound);
   }));
 
+  test('handles serving unresolvable package file', () => testbed.run(() async {
+    final Response response = await webAssetServer
+      .handleRequest(Request('GET', Uri.parse('http://foobar/packages/notpackage/file')));
+
+    expect(response.statusCode, HttpStatus.notFound);
+  }));
+
   test('serves /packages/<package>/<path> files as if they were '
        'package:<package>/<path> uris', () => testbed.run(() async {
     final Uri expectedUri = packages.resolve(