Do not append sourceMappingURL to map files (#26851)

* Do not append `sourceMappingURL` to map files

* apply recs

build-system/compile/compile.js

@@ -22,6 +22,7 @@ const gap = require('gulp-append-prepend');
 const gulp = require('gulp');
 const gulpIf = require('gulp-if');
 const nop = require('gulp-nop');
+const pathModule = require('path');
 const rename = require('gulp-rename');
 const sourcemaps = require('gulp-sourcemaps');
 const {
@@ -117,6 +118,13 @@ function compile(
   options,
   timeInfo
 ) {
+  function shouldAppendSourcemappingURLText(file) {
+    // Do not append sourceMappingURL if its a sourcemap
+    return (
+      pathModule.extname(file.path) !== '.map' && options.esmPassCompilation
+    );
+  }
+
   const hideWarningsFor = [
     'third_party/amp-toolbox-cache-url/',
     'third_party/caja/',
@@ -401,7 +409,7 @@ function compile(
         .pipe(sourcemaps.write('.'))
         .pipe(
           gulpIf(
-            !!argv.esm,
+            shouldAppendSourcemappingURLText,
             gap.appendText(`\n//# sourceMappingURL=${outputFilename}.map`)
           )
         )