JS: when both clauses of if become empty during optimization, remove if entirely. Make condition and then clause of JsIf non-nullable. Fix #KT-13912

js/js.tests/test/org/jetbrains/kotlin/js/test/optimizer/BasicOptimizerTest.kt

@@ -108,6 +108,11 @@ abstract class BasicOptimizerTest(private var basePath: String) {
                     }
                     super.visitIf(x)
                 }
+
+                override fun visitLabel(x: JsLabel) {
+                    x.synthetic = isSyntheticId(x.name.ident)
+                    super.visitLabel(x)
+                }
             }.accept(stmt)
         }
     }
@@ -119,7 +124,6 @@ abstract class BasicOptimizerTest(private var basePath: String) {
 
     private fun isSyntheticId(id: String) = id.startsWith("$")
 
-
     private fun astToString(ast: List<JsStatement>): String {
         val output = TextOutputImpl()
         val visitor = JsSourceGenerationVisitor(output, null)

js/js.tests/test/org/jetbrains/kotlin/js/test/optimizer/RedundantLabelRemovalTest.kt

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2010-2016 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jetbrains.kotlin.js.test.optimizer
+
+import org.junit.Test
+
+class RedundantLabelRemovalTest : BasicOptimizerTest("redundant-label-removal") {
+    @Test fun emptyIfConditionPreserved() = box()
+
+    @Test fun ifWithEmptyThenAndNoElse() = box()
+}