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 #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
 -----------------