asset-manifest.json generated by CRA2 is not useful

Created by: pthorn

As mentioned in the discussion in #5306, asset-manifest.json generated by CRA2 is less than useful. As an example, this is asset-manifest.json as generated by yarn build for my project:

{
   "main.css": "/static/css/main.7263860d.chunk.css",
   "main.js": "/static/js/main.1d09f064.chunk.js",
   "main.js.map": "/static/js/main.1d09f064.chunk.js.map",
   "static/css/1.a7cf09a2.chunk.css": "/static/css/1.a7cf09a2.chunk.css",
   "static/js/1.f5aeaa31.chunk.js": "/static/js/1.f5aeaa31.chunk.js",
   "static/js/1.f5aeaa31.chunk.js.map": "/static/js/1.f5aeaa31.chunk.js.map",
   "runtime~main.js": "/static/js/runtime~main.229c360f.js",
   "runtime~main.js.map": "/static/js/runtime~main.229c360f.js.map",
   "static/css/1.a7cf09a2.chunk.css.map": "/static/css/1.a7cf09a2.chunk.css.map",
   "static/css/main.7263860d.chunk.css.map": "/static/css/main.7263860d.chunk.css.map",
   "index.html": "/index.html",
   "precache-manifest.981d9ec8b3a8232e0184126d691edb97.js": "/precache-manifest.981d9ec8b3a8232e0184126d691edb97.js",
   "service-worker.js": "/service-worker.js"
 }

It was quite an unpleasant surprise when I upgraded to CRA2. Here are some obvious problems with this file:

• it contains no information on which files are chunks of the same package
• there is no way to identify chunks (except the 'main' chunk) by key (random strings in keys? seriously?)
• there is absolutely no information on the order in which chunks are to be loaded

It can of course still be parsed by pattern matching file names but there are obvious disadvantages:

• any such parsing will rely on heuristics
• parsing will break if file naming scheme is ever changed
• what is the point of having this file at all? you could listdir() the build directory and obtain basically the same information

Here is a suggested asset-manifest.json format that could make it usable:

{
   "main.js": [
       "/static/js/1.f5aeaa31.chunk.js",  // <-- order is significant
       "/static/js/main.1d09f064.chunk.js",
   ],
   "main.js.map": [
       "static/js/1.f5aeaa31.chunk.js.map",   // <-- same order as in "main.js"
       "/static/js/main.1d09f064.chunk.js.map",
    ]
   "runtime~main.js": "/static/js/runtime~main.229c360f.js",
   "runtime~main.js.map": "/static/js/runtime~main.229c360f.js.map",
   "precache-manifest.js": "/precache-manifest.981d9ec8b3a8232e0184126d691edb97.js",
   "service-worker.js": "/service-worker.js"
   "main.css": [
       "/static/css/1.a7cf09a2.chunk.css"
       "/static/css/main.7263860d.chunk.css"
    ],
   "main.css.map": [
       "/static/css/1.a7cf09a2.chunk.css.map"
       "/static/css/main.7263860d.chunk.css.map"
    ],
 }