[JS IR] Add separate key to enable extensions in external interfaces

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

@@ -9,6 +9,7 @@ import org.jetbrains.kotlin.cli.common.arguments.K2JsArgumentConstants.*
 import org.jetbrains.kotlin.cli.common.messages.CompilerMessageSeverity
 import org.jetbrains.kotlin.cli.common.messages.MessageCollector
 import org.jetbrains.kotlin.config.ApiVersion
+import org.jetbrains.kotlin.config.LanguageFeature
 import org.jetbrains.kotlin.config.LanguageVersion
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 
@@ -214,6 +215,12 @@ class K2JSCompilerArguments : CommonCompilerArguments() {
     )
     var friendModules: String? by NullableStringFreezableVar(null)
 
+    @Argument(
+        value = "-Xenable-extension-functions-in-externals",
+        description = "Enable extensions functions members in external interfaces"
+    )
+    var extensionFunctionsInExternals: Boolean by FreezableVar(false)
+
     @Argument(value = "-Xmetadata-only", description = "Generate *.meta.js and *.kjsm files only")
     var metadataOnly: Boolean by FreezableVar(false)
 
@@ -241,6 +248,14 @@ class K2JSCompilerArguments : CommonCompilerArguments() {
             )
         }
     }
+
+    override fun configureLanguageFeatures(collector: MessageCollector): MutableMap<LanguageFeature, LanguageFeature.State> {
+        return super.configureLanguageFeatures(collector).apply {
+            if (extensionFunctionsInExternals) {
+                this[LanguageFeature.JsEnableExtensionFunctionInExternals] = LanguageFeature.State.ENABLED
+            }
+        }
+    }
 }
 
 fun K2JSCompilerArguments.isPreIrBackendDisabled(): Boolean =