Output .js files from wasm modules as ES6 modules. (#844)

Buildroot changes needed for https://github.com/flutter/engine/pull/52023

build/config/compiler/BUILD.gn

diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 1da7b04..d38a4cf 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -296,6 +296,8 @@
       # Reduces global namespace pollution.
       "-s",
       "MODULARIZE=1",
+      "-s",
+      "EXPORT_ES6",
 
       # Always produce a symbol map so that we can map crash traces
       "--emit-symbol-map",