Copy the asm object if we modify it in ASSERTIONS mode. That makes it…

… work if it is the ES6 exports object of a wasm module, which is read-only. Fixes #8678

emscripten.py

@@ -1684,6 +1684,15 @@ def create_the_global(metadata):
   assert(runtimeInitialized, 'you need to wait for the runtime to be ready (e.g. wait for main() to be called)');
   assert(!runtimeExited, 'the runtime was exited (use NO_EXIT_RUNTIME to keep it alive after main() exits)');'''
 
+# We want to modify `asm` in some cases, for assertion purposes, however in ES6 environments that
+# may be illegal as the wasm exports are an ES6 export object, which is read-only. To work around
+# that, we simply make a copy of `asm` - the overhead doesn't matter in an assertions build.
+COPY_ASM = '''
+var originalAsm = asm;
+Module['asm'] = asm = {};
+for (var x in originalAsm) asm[x] = originalAsm[x];
+'''
+
 
 def create_receiving(function_table_data, function_tables_defs, exported_implemented_functions, initializers):
   receiving = ''
@@ -1695,6 +1704,8 @@ def create_receiving(function_table_data, function_tables_defs, exported_impleme
     # some support code
     receiving_functions = [f for f in exported_implemented_functions if f not in ('_memcpy', '_memset', '_emscripten_replace_memory', '__start_module')]
 
+    receiving = COPY_ASM
+
     wrappers = []
     for name in receiving_functions:
       wrappers.append('''\
@@ -1703,15 +1714,10 @@ def create_receiving(function_table_data, function_tables_defs, exported_impleme
   return real_%(name)s.apply(null, arguments);
 };
 ''' % {'name': name, 'runtime_assertions': runtime_assertions})
-    receiving = '\n'.join(wrappers)
+    receiving = COPY_ASM + '\n'.join(wrappers)
 
   shared.Settings.MODULE_EXPORTS = module_exports = exported_implemented_functions + function_tables(function_table_data)
 
-  # Currently USE PTHREADS + EXPORT ES6 doesn't work with the previously generated assertions.
-  # ES6 modules disallow re-assigning read-only properties.
-  if shared.Settings.USE_PTHREADS and shared.Settings.EXPORT_ES6 and shared.Settings.ASSERTIONS and receiving:
-    receiving = "assert(!ENVIRONMENT_IS_PTHREAD, 'Error: USE_PTHREADS and EXPORT_ES6 requires ASSERTIONS=0');\n" + receiving
-
   if not shared.Settings.SWAPPABLE_ASM_MODULE:
     if shared.Settings.DECLARE_ASM_MODULE_EXPORTS:
       imported_exports = [s for s in module_exports if s not in initializers]
@@ -2462,6 +2468,7 @@ def create_receiving_wasm(exports):
     runtime_assertions = RUNTIME_ASSERTIONS
     # assert on the runtime being in a valid state when calling into compiled code. The only exceptions are
     # some support code
+    receiving.append(COPY_ASM)
     for e in exports:
       receiving.append('''\
 var real_%(mangled)s = asm["%(e)s"];