Update the whitespace bundler to support ECMASCRIPT_NEXT features.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=187393364

src/com/google/javascript/jscomp/transpile/BaseTranspiler.java

@@ -156,6 +156,7 @@ public static class EsmToCjsCompilerSupplier extends CompilerSupplier {
     @Override
     public CompileResult compile(Path path, String code) {
       CompilerOptions options = new CompilerOptions();
+      options.setLanguageIn(LanguageMode.ECMASCRIPT_NEXT);
       options.setSourceMapOutputPath("/dev/null");
       options.setSourceMapIncludeSourcesContent(true);
       options.setPrettyPrint(true);