Serve packages uris in flutter_tools dev web server (#48743)

* support mapping /packages/<package>/<path> requests to package:<package>/<path> uris in the web device file server

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

@@ -11,6 +11,8 @@ import 'package:flutter_tools/src/base/io.dart';
 import 'package:flutter_tools/src/convert.dart';
 import 'package:flutter_tools/src/web/devfs_web.dart';
 import 'package:mockito/mockito.dart';
+import 'package:package_config/discovery.dart';
+import 'package:package_config/packages.dart';
 import 'package:platform/platform.dart';
 import 'package:flutter_tools/src/globals.dart' as globals;
 
@@ -36,6 +38,11 @@ void main() {
   WebAssetServer webAssetServer;
   MockPlatform windows;
   MockPlatform linux;
+  Packages packages;
+
+  setUpAll(() async {
+    packages = await loadPackagesFile(Uri.base.resolve('.packages'));
+  });
 
   setUp(() {
     windows = MockPlatform();
@@ -60,7 +67,8 @@ void main() {
       when(response.close()).thenAnswer((Invocation invocation) async {
         closeCompleter.complete();
       });
-      webAssetServer = WebAssetServer(mockHttpServer, onError: (dynamic error, StackTrace stackTrace) {
+      webAssetServer = WebAssetServer(
+          mockHttpServer, packages, onError: (dynamic error, StackTrace stackTrace) {
         closeCompleter.completeError(error, stackTrace);
       });
     });
@@ -291,6 +299,23 @@ void main() {
     verify(response.statusCode = HttpStatus.notFound).called(1);
   }));
 
+  test('serves /packages/<package>/<path> files as if they were '
+       'package:<package>/<path> uris', () => testbed.run(() async {
+    final Uri expectedUri = packages.resolve(
+        Uri.parse('package:flutter_tools/foo.dart'));
+    final File source = globals.fs.file(globals.fs.path.fromUri(expectedUri))
+      ..createSync(recursive: true)
+      ..writeAsBytesSync(<int>[1, 2, 3]);
+    when(request.uri).thenReturn(
+        Uri.parse('http:///packages/flutter_tools/foo.dart'));
+    requestController.add(request);
+    await closeCompleter.future;
+
+    verify(headers.add('Content-Length', source.lengthSync())).called(1);
+    verify(headers.add('Content-Type', 'application/octet-stream')).called(1);
+    verify(response.addStream(any)).called(1);
+  }));
+
   test('calling dispose closes the http server', () => testbed.run(() async {
     await webAssetServer.dispose();