Use babel for transpiling rather than closure compiler

We recently switch from using closure-compiler with `WHITESPACE_ONLY` to
closure-compiler with `SIMPLE_OPTIMIZATIONS`.  However this had the
negative side effect that all input need to be free of closure compiler
warnings, and this turned out not to be practical for all users.
See emscripten-core#20810 for more on this

When selecting a transpilation tool use I also evaluated `swx` (written
in rust) and `esbuild` (written in go).  `esbuild` was rejected
because the simply don't support ES5
(evanw/esbuild#297).  `swx` was rejected
because it almost doubled the side of our `node_modules` directory by
adding two 50mb binary files.

.flake8

@@ -1,6 +1,7 @@
 [flake8]
 ignore = E111,E114,E501,E261,E266,E121,E402,E241,W504,E741,B011,B023,U101
 exclude =
+ ./node_modules/, # third-party code
  ./third_party/, # third-party code
  ./tools/filelock.py, # third-party code
  ./tools/scons/, # third-party code

ChangeLog.md

@@ -20,20 +20,25 @@ See docs/process.md for more on how version tagging works.
 
 3.1.51 (in development)
 -----------------------
+- Transpilation to support older environments/browsers is now performed by babel
+  rather than closure compiler.  This means that folks targeting older browsers
+  (e.g. `-sLEGACY_VM_SUPPORT`) do not need to ensure their code is closure
+  compliant. (#20879)
 - Breaking change: Using the `*glGetProcAddress()` family of functions now
   requires passing a linker flag -sGL_ENABLE_GET_PROC_ADDRESS. This prevents
   ports of native GL renderers from later accidentally attempting to activate
   "dormant" features if web browser implementations gain new WebGL extensions in
   the future, which `*glGetProcAddress()` is not able to support. (#20802)
-- Added Hi DPI support to GLFW. When enabled, GLFW automatically accounts for the
-  `devicePixelRatio` browser property and changes the size of the canvas accordingly
-  (including dynamically if the canvas is moved from a 4k screen to a 2k screen and 
-  vice-versa). `glfwGetFramebufferSize` now properly returns the canvas size in pixels, 
-  while `glfwGetWindowSize` returns the canvas size is screen size. By default,
-  this feature is disabled. You can enable it before creating a window by calling 
-  `glfwWindowHint(GLFW_SCALE_TO_MONITOR, GLFW_TRUE)`. You can also 
-  dynamically change it after the window has been created by calling
-  `glfwSetWindowAttrib(window, GLFW_SCALE_TO_MONITOR, GLFW_TRUE)`. (#20584)
+- Added Hi DPI support to GLFW. When enabled, GLFW automatically accounts for
+  the `devicePixelRatio` browser property and changes the size of the canvas
+  accordingly (including dynamically if the canvas is moved from a 4K screen to
+  a 2K screen and vice-versa). `glfwGetFramebufferSize` now properly returns the
+  canvas size in pixels, while `glfwGetWindowSize` returns the canvas size is
+  screen size. By default, this feature is disabled. You can enable it before
+  creating a window by calling `glfwWindowHint(GLFW_SCALE_TO_MONITOR,
+  GLFW_TRUE)`. You can also dynamically change it after the window has been
+  created by calling `glfwSetWindowAttrib(window, GLFW_SCALE_TO_MONITOR,
+  GLFW_TRUE)`. (#20584)
 
 3.1.50 - 11/29/23
 -----------------