Set the RescopeGlobalSymbols FeatureSet to ES8_MODULES.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=179851521

Author: lauraharker

src/com/google/javascript/jscomp/DefaultPassConfig.java

@@ -2506,7 +2506,7 @@ protected CompilerPass create(AbstractCompiler compiler) {
 
     @Override
     protected FeatureSet featureSet() {
-      return ES5;
+      return ES8_MODULES;
     }
   };