JS backend: fix dangling else issue in generated code for JsToStringGenerationVisitor

JS backend: fix KT-5253: when with if inside

 #KT-5253 Fixed

js/js.dart-ast/src/com/google/dart/compiler/backend/js/JsToStringGenerationVisitor.java

@@ -558,10 +558,13 @@ public class JsToStringGenerationVisitor extends JsVisitor {
         accept(x.getIfExpression());
         rightParen();
         JsStatement thenStmt = x.getThenStatement();
+        JsStatement elseStatement = x.getElseStatement();
+        if (elseStatement != null && thenStmt instanceof JsIf && ((JsIf)thenStmt).getElseStatement() == null) {
+            thenStmt = new JsBlock(thenStmt);
+        }
         nestedPush(thenStmt);
         accept(thenStmt);
         nestedPop(thenStmt);
-        JsStatement elseStatement = x.getElseStatement();
         if (elseStatement != null) {
             if (needSemi) {
                 semi();

js/js.tests/test/org/jetbrains/k2js/test/semantics/WhenTest.java

@@ -108,4 +108,8 @@ public final class WhenTest extends AbstractExpressionTest {
     public void testWhenWithOneStmWhen() throws Exception {
         fooBoxTest();
     }
+
+    public void testIfInWhenDanglingElseIssue() throws Exception {
+        checkFooBoxIsOk();
+    }
 }

js/js.translator/testData/expression/when/cases/ifInWhenDanglingElseIssue.kt

@@ -0,0 +1,27 @@
+// http://youtrack.jetbrains.com/issue/KT-5253
+// JS: generated wrong code when use `if` inside `when`
+
+package foo
+
+
+fun test(caseId: Int, value: Int, expected: Int) {
+    var actual: Int = 0
+    when (caseId) {
+        2 -> if (value < 0) actual = -value
+        3 -> actual = if (value < 0) {-value} else value
+        else -> throw Exception("Unexpected case: $caseId")
+    }
+
+    if (expected != actual) throw Exception("expected = $expected, actual = $actual")
+}
+
+
+fun box(): String {
+
+    test(2, 33, 0)
+    test(2, -1, 1)
+    test(3, 23, 23)
+    test(3, -3, 3)
+
+    return "OK"
+}