Gradle, JS, webpack: source maps support

#KT-31013

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

@@ -13,7 +13,6 @@ import org.gradle.deployment.internal.DeploymentHandle
 import org.gradle.deployment.internal.DeploymentRegistry
 import org.gradle.process.internal.ExecHandle
 import org.gradle.process.internal.ExecHandleFactory
-import org.gradle.process.internal.ExecHandleState
 import org.jetbrains.kotlin.gradle.plugin.KotlinCompilationToRunnableFiles
 import org.jetbrains.kotlin.gradle.targets.js.npm.npmProject
 import org.jetbrains.kotlin.gradle.tasks.Kotlin2JsCompile
@@ -60,6 +59,9 @@ open class KotlinWebpack : DefaultTask() {
     @Input
     var bin: String = "webpack"
 
+    @Input
+    var sourceMaps: Boolean = true
+
     @Input
     @Optional
     var devServer: KotlinWebpackConfig.DevServer? = null
@@ -75,7 +77,8 @@ open class KotlinWebpack : DefaultTask() {
             outputPath,
             configDirectory,
             if (report) reportDir else null,
-            devServer
+            devServer,
+            sourceMaps = sourceMaps
         )
     )
 
@@ -117,6 +120,10 @@ open class KotlinWebpack : DefaultTask() {
                 runtimeOnly(npm("webpack", "4.29.6"))
                 runtimeOnly(npm("webpack-cli", "3.3.0"))
                 runtimeOnly(npm("webpack-bundle-analyzer", "3.3.2"))
+
+                // for source map support only
+                runtimeOnly(npm("source-map-loader", "0.2.4"))
+                runtimeOnly(npm("source-map-support", "0.5.12"))
             }
 
             project.createOrRegisterTask<KotlinWebpack>("webpack") {

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

@@ -21,7 +21,8 @@ class KotlinWebpackConfig(
     val configDirectory: File,
     val reportDir: File?,
     var devServer: DevServer? = null,
-    val showProgress: Boolean = false
+    val showProgress: Boolean = false,
+    val sourceMaps: Boolean = false
 ) {
     @Suppress("unused")
     data class BundleAnalyzerPlugin(
@@ -54,7 +55,7 @@ class KotlinWebpackConfig(
             """  
 var config = {
   mode: 'development',
-  entry: '${entry.canonicalPath}',
+  entry: ['${entry.canonicalPath}'],
   output: {
     path: '${outputPath.canonicalPath}',
     filename: '${entry.name}'
@@ -66,12 +67,29 @@ var config = {
   },
   plugins: [],
   module: {
-    rules: []
+    rules: [
-  }
+      {
-};"""
+        test: /\.js${'$'}/,
+        use: ["source-map-loader"],
+        enforce: "pre"
+      }
+    ]
+  },
+  devtool: 'eval-source-map'
+};""".trim()
         )
         appendln()
 
+        if (sourceMaps) {
+            //language=JavaScript 1.8
+            appendln("""
+// source maps
+config.entry.push('source-map-support/browser-source-map-support.js');
+config.module.rules.push({test: /\.js${'$'}/, use: ['source-map-loader'], enforce: 'pre'});
+                """.trim())
+            appendln()
+        }
+
         if (devServer != null) {
             appendln("// dev server")
             appendln("config.devServer = ${json(devServer!!)};")
@@ -80,7 +98,7 @@ var config = {
 
         if (reportDir != null) {
             val reportBasePath = "${reportDir.canonicalPath}/${entry.name}"
-            val config = KotlinWebpackConfig.BundleAnalyzerPlugin(
+            val config = BundleAnalyzerPlugin(
                 "static",
                 "$reportBasePath.report.html",
                 false,