JS: fix error in generation of if/else statements in some cases

See KT-19495

js/js.ast/src/org/jetbrains/kotlin/js/backend/JsToStringGenerationVisitor.java

@@ -644,7 +644,7 @@ public class JsToStringGenerationVisitor extends JsVisitor {
 
         JsStatement thenStmt = x.getThenStatement();
         JsStatement elseStatement = x.getElseStatement();
-        if (elseStatement != null && thenStmt instanceof JsIf && ((JsIf)thenStmt).getElseStatement() == null) {
+        if (elseStatement != null && isIfWithoutElse(thenStmt)) {
             thenStmt = new JsBlock(thenStmt);
         }
         nestedPush(thenStmt);
@@ -680,6 +680,18 @@ public class JsToStringGenerationVisitor extends JsVisitor {
         }
     }
 
+    private static boolean isIfWithoutElse(@NotNull JsStatement statement) {
+        while (statement instanceof JsIf) {
+            JsIf ifStatement = (JsIf) statement;
+            if (ifStatement.getElseStatement() == null) {
+                return true;
+            }
+            statement = ifStatement.getElseStatement();
+        }
+
+        return false;
+    }
+
     @Override
     public void visitInvocation(@NotNull JsInvocation invocation) {
         pushSourceInfo(invocation.getSource());