Remove magic === Body === tag. NFC (#23877)

Author: sbc100

src/preamble.js

@@ -1085,4 +1085,4 @@ function getCompilerSetting(name) {
 var asyncifyStubs = {};
 #endif
 
-// === Body ===
+<<< EM_JS_CODE >>>

src/preamble_minimal.js

@@ -55,4 +55,4 @@ var runtimeExited = false;
 #include "runtime_init_memory.js"
 #endif // IMPORTED_MEMORY
 
-// === Body ===
+<<< EM_JS_CODE >>>

test/other/test_unoptimized_code_size.js.size

@@ -1 +1 @@
-52639
+52624

test/other/test_unoptimized_code_size_no_asserts.js.size

@@ -1 +1 @@
-27521
+27506

test/other/test_unoptimized_code_size_strict.js.size

@@ -1 +1 @@
-50927
+50912

tools/emscripten.py

@@ -414,15 +414,12 @@ def emscript(in_wasm, out_wasm, outfile_js, js_syms, finalize=True, base_metadat
     pre = apply_static_code_hooks(forwarded_json, pre)
 
   asm_const_pairs = ['%s: %s' % (key, value) for key, value in asm_consts]
-  extra_code = ''
+  em_js_code = ''
   if asm_const_pairs or settings.MAIN_MODULE:
-    extra_code += 'var ASM_CONSTS = {\n  ' + ',  \n '.join(asm_const_pairs) + '\n};\n'
+    em_js_code += 'var ASM_CONSTS = {\n  ' + ',  \n '.join(asm_const_pairs) + '\n};\n'
   if em_js_funcs:
-    extra_code += '\n'.join(em_js_funcs) + '\n'
-  if extra_code:
-    pre = pre.replace(
-      '// === Body ===\n',
-      '// === Body ===\n\n' + extra_code + '\n')
+    em_js_code += '\n'.join(em_js_funcs) + '\n'
+  pre = shared.do_replace(pre, '<<< EM_JS_CODE >>>', em_js_code)
 
   if base_metadata:
     function_exports = base_metadata.function_exports