Enable InlineFunctions for language_out = ES2015+

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=182834207

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

@@ -2814,8 +2814,7 @@ protected CompilerPass create(AbstractCompiler compiler) {
 
         @Override
         public FeatureSet featureSet() {
-          // TODO(b/69850796): Switch this back to ES8_MODULES.
-          return ES5;
+          return ES8_MODULES;
         }
       };