Fix code coverage line mapping

When checking the code coverage report, it was noticed that the line numbers were off.
It was due to the fact that the files used for coverage were the transpiled ones,
when the ones used by Codecov were the original ones.
So this patches adds the source maps to the transpiled files, and also updates
the license header in the original files in using a babel plugin in order
to make sure the line numbers are correct.
As a side effect of this work, it's now possible to have the correct line
numbers in the stack traces when running tests with the transpiled files.

Author: calixteman

gulpfile.mjs

@@ -40,6 +40,7 @@ import { preprocess } from "./external/builder/builder.mjs";
 import relative from "metalsmith-html-relative";
 import rename from "gulp-rename";
 import replace from "gulp-replace";
+import sourcemaps from "gulp-sourcemaps";
 import stream from "stream";
 import TerserPlugin from "terser-webpack-plugin";
 import Vinyl from "vinyl";
@@ -1526,27 +1527,13 @@ gulp.task("types", function (done) {
 });
 
 function buildLibHelper(bundleDefines, inputStream, outputDir) {
-  function preprocessLib(content) {
-    const skipBabel = bundleDefines.SKIP_BABEL;
-    content = babel.transform(content, {
-      sourceType: "module",
-      presets: skipBabel
-        ? undefined
-        : [
-            [
-              "@babel/preset-env",
-              { ...BABEL_PRESET_ENV_OPTS, loose: false, modules: false },
-            ],
-          ],
-      plugins: [[babelPluginPDFJSPreprocessor, ctx]],
-      targets: BABEL_TARGETS,
-    }).code;
-    content = content.replaceAll(
-      /(\sfrom\s".*?)(?:\/src)(\/[^"]*"?;)$/gm,
-      (all, prefix, suffix) => prefix + suffix
-    );
-    return licenseHeaderLibre + content;
-  }
+  const licenseHeader = fs
+    .readFileSync("./src/license_header.js")
+    .toString()
+    .split("\n")
+    .slice(1, -2)
+    .map(line => line.replace(/^\s*\*\s?/, ""));
+
   const ctx = {
     rootPath: __dirname,
     defines: bundleDefines,
@@ -1564,12 +1551,82 @@ function buildLibHelper(bundleDefines, inputStream, outputDir) {
       "web-null_l10n": "../web/genericl10n.js",
     },
   };
-  const licenseHeaderLibre = fs
-    .readFileSync("./src/license_header_libre.js")
-    .toString();
-  return inputStream
-    .pipe(transform("utf8", preprocessLib))
-    .pipe(gulp.dest(outputDir));
+  const enableSourceMaps = bundleDefines.TESTING;
+
+  function preprocessLib(file, _enc, callback) {
+    const skipBabel = bundleDefines.SKIP_BABEL;
+
+    if (file.isNull()) {
+      return callback(null, file);
+    }
+
+    if (file.isStream()) {
+      return callback(new Error("Streaming not supported"));
+    }
+
+    try {
+      // Calculate where the output file will be
+      const outputFilePath = path.join(__dirname, outputDir, file.relative);
+      const outputFileDir = path.dirname(outputFilePath);
+      // Calculate relative path from output directory to source file
+      const relativeSourcePath = path.relative(outputFileDir, file.path);
+
+      const result = babel.transform(file.contents.toString(), {
+        sourceType: "module",
+        presets: skipBabel
+          ? undefined
+          : [
+              [
+                "@babel/preset-env",
+                { ...BABEL_PRESET_ENV_OPTS, loose: false, modules: false },
+              ],
+            ],
+        plugins: [
+          [babelPluginPDFJSPreprocessor, ctx],
+          [
+            "add-header-comment",
+            {
+              header: licenseHeader,
+            },
+          ],
+        ],
+        targets: BABEL_TARGETS,
+        sourceMaps: enableSourceMaps,
+        sourceFileName: relativeSourcePath,
+      });
+
+      let code = result.code;
+      code = code.replaceAll(
+        /(\sfrom\s".*?)(?:\/src)(\/[^"]*"?;)$/gm,
+        (all, prefix, suffix) => prefix + suffix
+      );
+
+      file.contents = Buffer.from(code);
+      // Attach the source map to the file for gulp-sourcemaps
+      if (result.map) {
+        file.sourceMap = result.map;
+      }
+
+      return callback(null, file);
+    } catch (err) {
+      return callback(err);
+    }
+  }
+
+  let pipeline = inputStream;
+  if (enableSourceMaps) {
+    pipeline = pipeline.pipe(sourcemaps.init({ loadMaps: true }));
+  }
+  pipeline = pipeline.pipe(
+    new stream.Transform({
+      objectMode: true,
+      transform: preprocessLib,
+    })
+  );
+  if (enableSourceMaps) {
+    pipeline = pipeline.pipe(sourcemaps.write("."));
+  }
+  return pipeline.pipe(gulp.dest(outputDir));
 }
 
 function buildLib(defines, dir) {
@@ -1956,6 +2013,7 @@ gulp.task(
         jasmineProcess = spawn("node", options, { stdio: "inherit" });
       } else {
         const options = [
+          "--enable-source-maps",
           "node_modules/jasmine/bin/jasmine",
           "JASMINE_CONFIG_PATH=test/unit/clitests.json",
         ];