Fix build on Windows

based on scratchfoundation#2138

.gitignore

@@ -44,3 +44,5 @@ python_compressed.js
 /*compiler*.jar
 /local_blockly_compressed_vertical.js
 /chromedriver
+# --flagfiles used on Windows
+/*.config

README.md

@@ -6,6 +6,12 @@
 
 ## Local development
 
+Requires Node.js (`node`), Python (`python3`), and Java (`java`). It is known to work in these environments but should work in others:
+
+ - Windows 10, Python 3.11.7 (Microsoft Store), Node.js 20.10.0 (nodejs.org installer), Java 11 (Temurin-11.0.21+9)
+
+Python 3.12 currently does not work.
+
 Install dependencies:
 
 ```sh
@@ -20,7 +26,7 @@ To build, run:
 npm run prepublish
 ```
 
-requires Python (2 or 3). scratch-gui development server must be restarted to update linked scratch-blocks.
+scratch-gui development server must be restarted to update linked scratch-blocks.
 
 <!--
 #### Scratch Blocks is a library for building creative computing interfaces.

build.py

@@ -36,7 +36,7 @@
 
 import sys
 
-import errno, glob, json, os, re, subprocess, threading, codecs, functools
+import errno, glob, json, os, re, subprocess, threading, codecs, functools, platform
 
 if sys.version_info[0] == 2:
   import httplib
@@ -333,9 +333,21 @@ def do_compile_local(self, params, target_filename):
       for group in [[CLOSURE_COMPILER_NPM], dash_args]:
         args.extend(filter(lambda item: item, group))
 
+      # On Windows, the command line is too long, so we save the arguments to a file instead
+      use_flagfile = platform.system() == "Windows"
+      if platform.system() == "Windows":
+        flagfile_name = target_filename + ".config"
+        with open(flagfile_name, "w") as f:
+          # \ needs to be escaped still
+          f.write(" ".join(args[1:]).replace("\\", "\\\\"))
+        args = [CLOSURE_COMPILER_NPM, "--flagfile", flagfile_name]
+
       proc = subprocess.Popen(args, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
       (stdout, stderr) = proc.communicate()
 
+      if use_flagfile:
+        os.remove(flagfile_name)
+
       # Build the JSON response.
       filesizes = [os.path.getsize(value) for (arg, value) in params if arg == "js_file"]
       return dict(