Write flutter_assets directory on Windows (#13564)

packages/flutter_tools/lib/src/flx.dart

@@ -140,6 +140,10 @@ Future<List<String>> assemble({
   ensureDirectoryExists(outputPath);
 
   printTrace('Encoding zip file to $outputPath');
+
+  // TODO(zarah): Remove the zipBuilder and write the files directly once FLX
+  // is deprecated.
+
   await zipBuilder.createZip(fs.file(outputPath), fs.directory(workingDirPath));
 
   printTrace('Built $outputPath.');

packages/flutter_tools/lib/src/zip.dart

@@ -33,16 +33,26 @@ class _ArchiveZipBuilder extends ZipBuilder {
   Future<Null> createZip(File outFile, Directory zipBuildDir) async {
     final Archive archive = new Archive();
 
+    if (zipBuildDir.existsSync())
+      zipBuildDir.deleteSync(recursive: true);
+    zipBuildDir.createSync(recursive: true);
+
     final Completer<Null> finished = new Completer<Null>();
     int count = entries.length;
     entries.forEach((String archivePath, DevFSContent content) {
       content.contentsAsBytes().then<Null>((List<int> data) {
         archive.addFile(new ArchiveFile.noCompress(archivePath, data.length, data));
+
+        final File file = fs.file(fs.path.join(zipBuildDir.path, archivePath));
+        file.parent.createSync(recursive: true);
+
+        file.writeAsBytes(data).then<Null>((File value) {
           count -= 1;
           if (count == 0)
             finished.complete();
         });
       });
+    });
     await finished.future;
 
     final List<int> zipData = new ZipEncoder().encode(archive);