diff --git a/utils/kernel-service/BUILD.gn b/utils/kernel-service/BUILD.gn
index 9f926e39a91..6587477d840 100644
--- a/utils/kernel-service/BUILD.gn
+++ b/utils/kernel-service/BUILD.gn
@@ -29,7 +29,10 @@ kernel_application_snapshot("kernel-service_snapshot") {
   ]
   training_args = [
     "--train",
-    "file:///" + rebase_path("../../pkg/compiler/lib/src/dart2js.dart"),
+    # Force triple-slashes both on Windows and otherwise.
+    # Becomes e.g. file:///full/path/to/file and "file:///C:/full/path/to/file.
+    # Without the ', "/"' part, on Linux it would get four slashes.
+    "file:///" + rebase_path("../../pkg/compiler/lib/src/dart2js.dart", "/"),
   ]
   if (dart_platform_bytecode) {
     training_args += [ "--bytecode" ]