Change as per David's comment.

build/config/compiler/BUILD.gn

diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 5eb7bef..d38a4cf 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -296,7 +296,8 @@
       # Reduces global namespace pollution.
       "-s",
       "MODULARIZE=1",
-      "-sEXPORT_ES6",
+      "-s",
+      "EXPORT_ES6",
 
       # Always produce a symbol map so that we can map crash traces
       "--emit-symbol-map",