Compile wasm modules as ES6 modules.

build/config/compiler/BUILD.gn

diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 1da7b04..5eb7bef 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -296,6 +296,7 @@
       # Reduces global namespace pollution.
       "-s",
       "MODULARIZE=1",
+      "-sEXPORT_ES6",
 
       # Always produce a symbol map so that we can map crash traces
       "--emit-symbol-map",