JS: in tests add verifier that checks whether no expression statements with only null expression exist

js/js.tests/test/org/jetbrains/kotlin/js/test/BasicTest.java

@@ -49,6 +49,7 @@ import org.jetbrains.kotlin.js.facade.TranslationResult;
 import org.jetbrains.kotlin.js.test.rhino.RhinoResultChecker;
 import org.jetbrains.kotlin.js.test.utils.DirectiveTestUtils;
 import org.jetbrains.kotlin.js.test.utils.JsTestUtils;
+import org.jetbrains.kotlin.js.test.utils.JsVerificationKt;
 import org.jetbrains.kotlin.js.translate.context.Namer;
 import org.jetbrains.kotlin.psi.KtFile;
 import org.jetbrains.kotlin.psi.KtPsiFactory;
@@ -249,6 +250,7 @@ public abstract class BasicTest extends KotlinTestWithEnvironment {
             String text = file.getText();
             DirectiveTestUtils.processDirectives(program, text);
         }
+        JsVerificationKt.verifyAst(program);
     }
 
     protected void runRhinoTests(

js/js.tests/test/org/jetbrains/kotlin/js/test/utils/jsVerification.kt

@@ -0,0 +1,37 @@
+/*
+ * 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.utils
+
+import com.google.dart.compiler.backend.js.ast.JsExpressionStatement
+import com.google.dart.compiler.backend.js.ast.JsNullLiteral
+import com.google.dart.compiler.backend.js.ast.JsProgram
+import com.google.dart.compiler.backend.js.ast.RecursiveJsVisitor
+import org.junit.Assert
+
+fun JsProgram.verifyAst() {
+    accept(object : RecursiveJsVisitor() {
+        override fun visitExpressionStatement(x: JsExpressionStatement) {
+            val expression = x.expression
+            if (expression is JsNullLiteral) {
+                Assert.fail("Expression statement contains `null` literal")
+            }
+            else {
+                super.visitExpressionStatement(x)
+            }
+        }
+    })
+}