JS: fix crash when js function contains for statement w/o initializer

Fix KT-20898

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

@@ -144,7 +144,7 @@ public class JsFor extends SourceInfoAwareJsNode implements JsStatement {
         if (initVars != null) {
             result = new JsFor(initVars.deepCopy(), conditionCopy, incrementalExprCopy, bodyCopy);
         } else {
-            result = new JsFor(initExpression.deepCopy(), conditionCopy, incrementalExprCopy, bodyCopy);
+            result = new JsFor(initExpression != null ? initExpression.deepCopy() : null, conditionCopy, incrementalExprCopy, bodyCopy);
         }
 
         return result.withMetadataFrom(this);

js/js.parser/src/com/google/gwt/dev/js/JsAstMapper.java

@@ -536,12 +536,11 @@ public class JsAstMapper {
             JsNode init = map(fromInit);
             JsExpression condition = mapOptionalExpression(fromTest);
             JsExpression increment = mapOptionalExpression(fromIncr);
-            assert (init != null);
             if (init instanceof JsVars) {
                 toFor = new JsFor((JsVars) init, condition, increment);
             }
             else {
-                assert (init instanceof JsExpression);
+                assert (init == null || init instanceof JsExpression);
                 toFor = new JsFor((JsExpression) init, condition, increment);
             }
 

js/js.serializer/src/org/jetbrains/kotlin/serialization/js/ast/JsAstDeserializer.kt

@@ -223,7 +223,7 @@ class JsAstDeserializer(program: JsProgram, private val sourceRoots: Iterable<Fi
                 JsFor(initVars, condition, increment, body)
             }
             else {
-                JsFor(initExpr!!, condition, increment, body)
+                JsFor(initExpr, condition, increment, body)
             }
         }
 

js/js.tests/test/org/jetbrains/kotlin/js/test/semantics/BoxJsTestGenerated.java

@@ -5480,6 +5480,12 @@ public class BoxJsTestGenerated extends AbstractBoxJsTest {
             doTest(fileName);
         }
 
+        @TestMetadata("forWithoutInit.kt")
+        public void testForWithoutInit() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("js/js.translator/testData/box/jsCode/forWithoutInit.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("function.kt")
         public void testFunction() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("js/js.translator/testData/box/jsCode/function.kt");

js/js.translator/testData/box/jsCode/forWithoutInit.kt

@@ -0,0 +1,20 @@
+// EXPECTED_REACHABLE_NODES: 1125
+// FILE: a.kt
+fun foo(n: Int): String = js("""
+    var result = "";
+
+    var i = 0;
+    for (; i < n; i++) {
+        result += i + ";"
+    }
+
+    return result;
+""")
+
+// FILE: b.kt
+// RECOMPILE
+fun box(): String {
+    val r = foo(3)
+    if (r != "0;1;2;") return "fail: $r"
+    return "OK"
+}