[JS IR] Add -Xgenerate-dts CLI argument

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JSCompilerArguments.kt

@@ -125,6 +125,12 @@ class K2JSCompilerArguments : CommonCompilerArguments() {
     @Argument(value = "-Xir-only", description = "Disables pre-IR backend")
     var irOnly: Boolean by FreezableVar(false)
 
+    @Argument(
+        value = "-Xgenerate-dts",
+        description = "Generate TypeScript declarations .d.ts file alongside JS file. Available in IR backend only."
+    )
+    var generateDts: Boolean by FreezableVar(false)
+
     @GradleOption(DefaultValues.BooleanTrueDefault::class)
     @Argument(value = "-Xtyped-arrays", description = "Translate primitive arrays to JS typed arrays")
     var typedArrays: Boolean by FreezableVar(true)

compiler/cli/cli-js/src/org/jetbrains/kotlin/cli/js/K2JsIrCompiler.kt

@@ -45,6 +45,7 @@ import org.jetbrains.kotlin.util.Logger
 import org.jetbrains.kotlin.utils.JsMetadataVersion
 import org.jetbrains.kotlin.utils.KotlinPaths
 import org.jetbrains.kotlin.utils.PathUtil
+import org.jetbrains.kotlin.utils.fileUtils.withReplacedExtensionOrNull
 import org.jetbrains.kotlin.utils.join
 import java.io.File
 import java.io.IOException
@@ -208,6 +209,10 @@ class K2JsIrCompiler : CLICompiler<K2JSCompilerArguments>() {
             )
 
             outputFile.writeText(compiledModule.jsCode)
+            if (arguments.generateDts) {
+                val dtsFile = outputFile.withReplacedExtensionOrNull(outputFile.extension, "d.ts")!!
+                dtsFile.writeText(compiledModule.tsDefinitions ?: error("No ts definitions"))
+            }
         }
 
         return OK