Warn when a pass is skipped because its incompatible with the current…

… language mode.

This will be a little noisy for the few projects that have --language_out=ES_2015 or higher, but will make it clearer which passes are being skipped which otherwise would run.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=158762960

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

@@ -279,7 +279,7 @@ class NamedPass implements CompilerPass {
     @Override
     public void process(Node externs, Node root) {
       if (!factory.featureSet().contains(compiler.getFeatureSet())) {
-        logger.fine(
+        logger.warning(
             "Skipping pass " + name
                 + "\nfactory features:  " + factory.featureSet()
                 + "\ncompiler features: " + compiler.getFeatureSet());