workaround for mangled web sdk source map packages (#39774)

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

@@ -262,7 +262,7 @@ class WebFs {
         return Response.ok(file.readAsBytesSync(), headers: <String, String>{
           'Content-Type': 'text/javascript',
         });
-      } else if (request.url.path.contains('dart_sdk')) {
+      } else if (request.url.path.endsWith('dart_sdk.js')) {
         final File file = fs.file(fs.path.join(
           artifacts.getArtifactPath(Artifact.flutterWebSdk),
           'kernel',
@@ -272,12 +272,29 @@ class WebFs {
         return Response.ok(file.readAsBytesSync(), headers: <String, String>{
           'Content-Type': 'text/javascript',
         });
+      } else if (request.url.path.endsWith('dart_sdk.js.map')) {
+        final File file = fs.file(fs.path.join(
+          artifacts.getArtifactPath(Artifact.flutterWebSdk),
+          'kernel',
+          'amd',
+          'dart_sdk.js.map',
+        ));
+        return Response.ok(file.readAsBytesSync());
       } else if (request.url.path.endsWith('.dart')) {
         // This is likely a sourcemap request. The first segment is the
         // package name, and the rest is the path to the file relative to
         // the package uri. For example, `foo/bar.dart` would represent a
         // file at a path like `foo/lib/bar.dart`. If there is no leading
         // segment, then we assume it is from the current package.
+
+        // Handle sdk requests that have mangled urls from engine build.
+        if (request.url.path.contains('flutter_web_sdk')) {
+          // Note: the request is a uri and not a file path, so they always use `/`.
+          final String sdkPath = fs.path.joinAll(request.url.path.split('flutter_web_sdk/').last.split('/'));
+          final String webSdkPath = artifacts.getArtifactPath(Artifact.flutterWebSdk);
+          return Response.ok(fs.file(fs.path.join(webSdkPath, sdkPath)).readAsBytesSync());
+        }
+
         final String packageName = request.url.pathSegments.length == 1
           ? flutterProject.manifest.appName
           : request.url.pathSegments.first;