Fix a compiler crash with ES5 inputs when the language_out is ES6.

The ES6->ES5 passes were being enabled incorrectly based on the
assumption that was the only kind of transpilation.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=85117586

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

@@ -1634,9 +1634,13 @@ public LanguageMode getLanguageOut() {
     return languageOut;
   }
 
-  boolean needsConversion() {
+  /**
+   * @return whether we are currently transpiling from ES6 to a lower version.
+   */
+  boolean lowerFromEs6() {
     return languageOut != LanguageMode.NO_TRANSPILE
-        && languageIn != languageOut;
+        && languageIn.isEs6OrHigher()
+        && !languageOut.isEs6OrHigher();
   }
 
   @Override

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

@@ -216,11 +216,11 @@ protected List<PassFactory> getChecks() {
       checks.add(closureRewriteModule);
     }
 
-    if (options.needsConversion() || options.aggressiveVarCheck.isOn()) {
+    if (options.lowerFromEs6() || options.aggressiveVarCheck.isOn()) {
       checks.add(checkVariableReferences);
     }
 
-    if (options.needsConversion()) {
+    if (options.lowerFromEs6()) {
       checks.add(es6RenameVariablesInParamLists);
       checks.add(es6SplitVariableDeclarations);
       checks.add(es6ConvertSuper);
@@ -234,7 +234,7 @@ protected List<PassFactory> getChecks() {
       return checks;
     }
 
-    if (options.needsConversion()) {
+    if (options.lowerFromEs6()) {
       checks.add(es6RuntimeLibrary);
     }
 

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

@@ -2876,7 +2876,16 @@ public void testIssue937() {
     test(options, code, result);
   }
 
-
+  public void testES5toES6() throws Exception {
+    CompilerOptions options = createCompilerOptions();
+    options.setAllowEs6Out(true);
+    options.setLanguageIn(LanguageMode.ECMASCRIPT5_STRICT);
+    options.setLanguageOut(LanguageMode.ECMASCRIPT6_STRICT);
+    CompilationLevel.SIMPLE_OPTIMIZATIONS
+        .setOptionsForCompilationLevel(options);
+    String code = "f = function(c) { for (var i = 0; i < c.length; i++) {} };";
+    compile(options, code);
+  }
 
   public void testIssue787() {
     CompilerOptions options = createCompilerOptions();