[Gradle, JS] Default conpiler type instead of legacy

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/dsl/KotlinProjectExtension.kt

@@ -98,8 +98,10 @@ open class KotlinJsProjectExtension : KotlinSingleTargetExtension() {
 
     override lateinit var target: KotlinJsTargetDsl
 
+    internal lateinit var defaultCompilerType: JsCompilerType
+
     open fun js(
-        compiler: JsCompilerType = legacy,
+        compiler: JsCompilerType = defaultCompilerType,
         body: KotlinJsTargetDsl.() -> Unit
     ) {
         val target: KotlinJsTargetDsl = when (compiler) {
@@ -131,7 +133,7 @@ open class KotlinJsProjectExtension : KotlinSingleTargetExtension() {
 
     open fun js(
         body: KotlinJsTargetDsl.() -> Unit
-    ) = js(compiler = legacy, body = body)
+    ) = js(compiler = defaultCompilerType, body = body)
 
     @Deprecated("Use js instead", ReplaceWith("js(body)"))
     open fun target(body: KotlinJsTargetDsl.() -> Unit) = js(body)

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinProperties.kt

@@ -19,6 +19,8 @@ package org.jetbrains.kotlin.gradle.plugin
 import org.gradle.api.Project
 import org.jetbrains.kotlin.gradle.dsl.Coroutines
 import org.jetbrains.kotlin.gradle.dsl.NativeCacheKind
+import org.jetbrains.kotlin.gradle.plugin.JsCompilerType
+import org.jetbrains.kotlin.gradle.plugin.JsCompilerType.Companion.jsCompilerProperty
 import org.jetbrains.kotlin.gradle.targets.native.DisabledNativeTargetsReporter
 import org.jetbrains.kotlin.gradle.tasks.AbstractKotlinCompile
 import org.jetbrains.kotlin.gradle.tasks.Kotlin2JsCompile
@@ -168,6 +170,12 @@ internal class PropertiesProvider private constructor(private val project: Proje
     val jsDiscoverTypes: Boolean?
         get() = booleanProperty("kotlin.js.experimental.discoverTypes")
 
+    /**
+     * Use Kotlin/JS backend mode
+     */
+    val jsCompiler: JsCompilerType
+        get() = property(jsCompilerProperty)?.let { JsCompilerType.byArgument(it) } ?: JsCompilerType.legacy
+
     private fun propertyWithDeprecatedVariant(propName: String, deprecatedPropName: String): String? {
         val deprecatedProperty = property(deprecatedPropName)
         if (deprecatedProperty != null) {

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/KotlinJsPlugin.kt

@@ -12,6 +12,7 @@ import org.jetbrains.kotlin.gradle.dsl.KotlinJsProjectExtension
 import org.jetbrains.kotlin.gradle.dsl.kotlinExtension
 import org.jetbrains.kotlin.gradle.plugin.KotlinCompilation.Companion.MAIN_COMPILATION_NAME
 import org.jetbrains.kotlin.gradle.plugin.KotlinCompilation.Companion.TEST_COMPILATION_NAME
+import org.jetbrains.kotlin.gradle.plugin.PropertiesProvider
 import org.jetbrains.kotlin.gradle.plugin.configureDefaultVersionsResolutionStrategy
 import org.jetbrains.kotlin.gradle.plugin.mpp.KotlinJsSingleTargetPreset
 import org.jetbrains.kotlin.gradle.targets.js.ir.KotlinJsIrSingleTargetPreset
@@ -35,6 +36,7 @@ open class KotlinJsPlugin(
         kotlinExtension.apply {
             irPreset = KotlinJsIrSingleTargetPreset(project, kotlinPluginVersion)
             legacyPreset = KotlinJsSingleTargetPreset(project, kotlinPluginVersion)
+            defaultCompilerType = PropertiesProvider(project).jsCompiler
         }
 
         // Explicitly create configurations for main and test