[JS IR] Verify that we don't generate Unit_getInstance() statements

See: KT-51127

js/js.tests/test/org/jetbrains/kotlin/js/test/handlers/JsAstHandler.kt

@@ -6,12 +6,10 @@
 package org.jetbrains.kotlin.js.test.handlers
 
 import org.jetbrains.kotlin.ir.backend.js.transformers.irToJs.TranslationMode
-import org.jetbrains.kotlin.js.backend.ast.JsExpressionStatement
+import org.jetbrains.kotlin.js.backend.ast.*
-import org.jetbrains.kotlin.js.backend.ast.JsNullLiteral
-import org.jetbrains.kotlin.js.backend.ast.JsProgram
-import org.jetbrains.kotlin.js.backend.ast.RecursiveJsVisitor
 import org.jetbrains.kotlin.js.facade.TranslationResult
 import org.jetbrains.kotlin.js.testOld.utils.DirectiveTestUtils
+import org.jetbrains.kotlin.js.translate.utils.name
 import org.jetbrains.kotlin.test.TargetBackend
 import org.jetbrains.kotlin.test.backend.handlers.JsBinaryArtifactHandler
 import org.jetbrains.kotlin.test.model.BinaryArtifacts
@@ -35,21 +33,26 @@ class JsAstHandler(testServices: TestServices) : JsBinaryArtifactHandler(testSer
 
     private fun processJsProgram(program: JsProgram, psiFiles: List<String>, targetBackend: TargetBackend) {
         psiFiles.forEach { DirectiveTestUtils.processDirectives(program, it, targetBackend) }
-
+        program.verifyAst(targetBackend)
-        // TODO: For now the IR backend generates JS code that doesn't pass verification,
-        // TODO: so we temporarily disabled AST verification.
-        if (targetBackend == TargetBackend.JS) {
-            program.verifyAst()
-        }
     }
 
-    private fun JsProgram.verifyAst() {
+    private fun JsProgram.verifyAst(targetBackend: TargetBackend) {
         accept(object : RecursiveJsVisitor() {
             override fun visitExpressionStatement(x: JsExpressionStatement) {
-                when (x.expression) {
+                val expression = x.expression
-                    is JsNullLiteral -> testServices.assertions.fail { "Expression statement contains `null` literal" }
+
-                    else -> super.visitExpressionStatement(x)
+                // FIXME: The code generated by the IR BE contains null statements. Remove this check when that is fixed.
+                if (targetBackend == TargetBackend.JS) {
+                    if (expression is JsNullLiteral) {
+                        testServices.assertions.fail { "Expression statement contains `null` literal" }
+                    }
                 }
+
+                if (expression is JsInvocation && expression.name?.ident == "Unit_getInstance" && expression.arguments.isEmpty()) {
+                    testServices.assertions.fail { "Unit_getInstance() statements should be eliminated" }
+                }
+
+                super.visitExpressionStatement(x)
             }
         })
     }