Automated g4 rollback of changelist 97892108.

*** Reason for rollback ***

We don't really support --langauge_out=ES6 in any meaningful way

(and when we do, probably it will be from skipping transpilation rather than doing it twice).

*** Original change description ***

Allow --language_out=ES6

For now, what that means is that we transpile to ES5,
then do checks/optimizations, and then convert to ES6, where possible.

***
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=103891298

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

@@ -42,6 +42,13 @@ static void preprocess(CompilerOptions options) {
           + "remove_unused_prototype_properties to be turned on.");
     }
 
+    if (options.getLanguageOut().isEs6OrHigher()
+        && !options.skipNonTranspilationPasses && !options.skipTranspilationAndCrash) {
+      throw new InvalidOptionsException(
+          "ES6 is only supported for transpilation to a lower ECMAScript"
+          + " version. Set --language_out to ES3, ES5, or ES5_STRICT.");
+    }
+
     if (!options.inlineFunctions
         && options.maxFunctionSizeAfterInlining
         != CompilerOptions.UNLIMITED_FUN_SIZE_AFTER_INLINING) {

test/com/google/javascript/jscomp/IntegrationTest.java

@@ -2945,6 +2945,7 @@ public void testES5toES6() throws Exception {
     CompilerOptions options = createCompilerOptions();
     options.setLanguageIn(LanguageMode.ECMASCRIPT5_STRICT);
     options.setLanguageOut(LanguageMode.ECMASCRIPT6_STRICT);
+    options.setSkipTranspilationAndCrash(true);
     CompilationLevel.SIMPLE_OPTIMIZATIONS
         .setOptionsForCompilationLevel(options);
     String code = "f = function(c) { for (var i = 0; i < c.length; i++) {} };";
@@ -3219,6 +3220,7 @@ public void testIsEquivalentTo() {
   public void testEs6OutDoesntCrash() {
     CompilerOptions options = new CompilerOptions();
     options.setLanguageIn(LanguageMode.ECMASCRIPT6);
+    options.setSkipTranspilationAndCrash(true);
     test(options, "function f(x) { if (x) var x=5; }", "function f(x) { if (x) x=5; }");
   }