Don't fuse for-loop initializers with let/const in StatementFusion.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=178031548

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

@@ -137,7 +137,7 @@ private boolean isFusableControlStatement(Node n) {
         return n.hasChildren();
       case FOR:
         // Avoid cases where we have for(var x;_;_) { ....
-        return !n.getFirstChild().isVar();
+        return !NodeUtil.isNameDeclaration(n.getFirstChild());
       case FOR_IN:
         // Avoid cases where we have for(var x = foo() in a) { ....
         return !mayHaveSideEffects(n.getFirstChild());

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

@@ -28,7 +28,6 @@ public final class StatementFusionTest extends CompilerTestCase  {
   protected void setUp() throws Exception {
     super.setUp();
     favorsCommas = false;
-    setAcceptedLanguage(CompilerOptions.LanguageMode.ECMASCRIPT5);
   }
 
   @Override
@@ -87,17 +86,25 @@ public void testFuseIntoForIn1() {
   }
 
   public void testFuseIntoForIn2() {
+    // This test case causes a parse warning in ES5 strict out, but is a parse error in ES6+ out.
+    setAcceptedLanguage(CompilerOptions.LanguageMode.ECMASCRIPT5_STRICT);
     setExpectParseWarningsThisTest();
     fuseSame("a();for(var x = b() in y){}");
   }
 
-  public void testFuseIntoVanillaFor() {
+  public void testFuseIntoVanillaFor1() {
     fuse("a;b;c;for(;g;){}", "for(a,b,c;g;){}");
     fuse("a;b;c;for(d;g;){}", "for(a,b,c,d;g;){}");
     fuse("a;b;c;for(d,e;g;){}", "for(a,b,c,d,e;g;){}");
     fuseSame("a();for(var x;g;){}");
   }
 
+  public void testFuseIntoVanillaFor2() {
+    fuseSame("a;b;c;for(var d;g;){}");
+    fuseSame("a;b;c;for(let d;g;){}");
+    fuseSame("a;b;c;for(const d = 5;g;){}");
+  }
+
   public void testFuseIntoLabel() {
     fuse("a;b;c;label:for(x in y){}", "label:for(x in a,b,c,y){}");
     fuse("a;b;c;label:for(;g;){}", "label:for(a,b,c;g;){}");
@@ -146,7 +153,7 @@ public void testFavorComma5() {
     test("a;b;c;if(d){}d;e;f;if(g){}", "if(a,b,c,d){}if(d,e,f,g){}");
   }
 
-  public void testNoGlobalSchopeChanges() {
+  public void testNoGlobalScopeChanges() {
     testSame("a,b,c");
   }