webassembly: Move MP_JS_EPOCH init to library postset.

This eliminates the need for wrapper.js to run to set up the time.

Signed-off-by: Damien George <damien@micropython.org>

ports/webassembly/library.js

@@ -35,9 +35,10 @@ mergeInto(LibraryManager.library, {
         }
     },
 
-    mp_js_ticks_ms: function() {
-        return Date.now() - MP_JS_EPOCH;
-    },
+    // This string will be emitted directly into the output file by Emscripten.
+    mp_js_ticks_ms__postset: "var MP_JS_EPOCH = Date.now()",
+
+    mp_js_ticks_ms: () => Date.now() - MP_JS_EPOCH,
 
     mp_js_hook: function() {
         if (ENVIRONMENT_IS_NODE) {

ports/webassembly/wrapper.js

@@ -33,8 +33,6 @@ var mainProgram = function()
   mp_js_init_repl = Module.cwrap('mp_js_init_repl', 'null', ['null']);
   mp_js_process_char = Module.cwrap('mp_js_process_char', 'number', ['number'], {async: true});
 
-  MP_JS_EPOCH = Date.now();
-
   if (typeof window === 'undefined' && require.main === module) {
       var fs = require('fs');
       var heap_size = 128 * 1024;