diff --git a/tools/isobuild/import-scanner.js b/tools/isobuild/import-scanner.js
index dca37871a0..daa06dec37 100644
--- a/tools/isobuild/import-scanner.js
+++ b/tools/isobuild/import-scanner.js
@@ -249,19 +249,6 @@ export default class ImportScanner {
       // something plausible. #6411 #6383
       const absPath = pathJoin(this.sourceRoot, file.sourcePath);
 
-      const dotExt = "." + file.type;
-      const dataString = file.data.toString("utf8");
-      file.dataString = defaultExtensionHandlers[dotExt].call(
-        file,
-        dataString,
-        file.hash,
-      );
-
-      if (! (file.data instanceof Buffer) ||
-          file.dataString !== dataString) {
-        file.data = Buffer.from(file.dataString, "utf8");
-      }
-
       // This property can have values false, true, "dynamic" (which
       // indicates that the file has been imported, but only dynamically).
       file.imported = false;
@@ -302,9 +289,10 @@ export default class ImportScanner {
     const files = this.realPathToFiles[realPath];
     if (files && files.length > 0) {
       const firstFile = files[0];
+      const dataString = this._getDataString(firstFile);
       return {
         data: firstFile.data,
-        dataString: firstFile.dataString,
+        dataString: dataString,
         hash: firstFile.hash,
       };
     }
@@ -417,7 +405,7 @@ export default class ImportScanner {
       // plugin calling inputFile.addJavaScript multiple times for the
       // same source file (see discussion in #9176), with different target
       // paths, code, laziness, etc.
-      sourceFile.dataString += [
+      sourceFile.dataString = this._getDataString(sourceFile) + [
         "module.watch(require(" + JSON.stringify(relativeId) + "), {",
         '  "*": module.makeNsSetter(true)',
         "});",
@@ -437,6 +425,8 @@ export default class ImportScanner {
   // maps and updating all other properties appropriately. Once this
   // combination is done, oldFile should be kept and newFile discarded.
   _combineFiles(oldFile, newFile) {
+    const scanner = this;
+
     function checkProperty(name) {
       if (has(oldFile, name)) {
         if (! has(newFile, name)) {
@@ -469,8 +459,11 @@ export default class ImportScanner {
       const consumer = file.sourceMap &&
         new SourceMapConsumer(file.sourceMap);
       const node = consumer &&
-        SourceNode.fromStringWithSourceMap(file.dataString, consumer);
-      return node || file.dataString;
+        SourceNode.fromStringWithSourceMap(
+          scanner._getDataString(file),
+          consumer
+        );
+      return node || scanner._getDataString(file);
     }
 
     const {
@@ -728,7 +721,7 @@ export default class ImportScanner {
     }
 
     const result = findImportedModuleIdentifiers(
-      file.dataString,
+      this._getDataString(file),
       file.hash,
     );
 
@@ -913,6 +906,27 @@ export default class ImportScanner {
     return archMatches(this.bundleArch, "web.browser");
   }
 
+  _getDataString(file) {
+    if (typeof file.dataString === "string") {
+      return file.dataString;
+    }
+
+    const dotExt = "." + file.type;
+    const dataString = file.data.toString("utf8");
+    file.dataString = defaultExtensionHandlers[dotExt].call(
+      file,
+      dataString,
+      file.hash,
+    );
+
+    if (! (file.data instanceof Buffer) ||
+        file.dataString !== dataString) {
+      file.data = Buffer.from(file.dataString, "utf8");
+    }
+
+    return file.dataString;
+  }
+
   _readFile(absPath) {
     const contents = optimisticReadFile(absPath);
     const hash = optimisticHashOrNull(absPath);