JS: generate paths in source maps relative to .map file location

See KT-19818

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

@@ -461,11 +461,9 @@ open class Kotlin2JsCompile() : AbstractKotlinCompile<K2JSCompilerArguments>(),
 
         args.friendModules = friendDependency
 
-        args.sourceMapBaseDirs = source.orEmpty()
-                .asSequence()
-                .filterIsInstance<SourceDirectorySet>()
-                .flatMap { it.srcDirs.asSequence() }
-                .joinToString(File.pathSeparator) { it.absolutePath }
+        if (args.sourceMapBaseDirs == null && !args.sourceMapPrefix.isNullOrEmpty()) {
+            args.sourceMapBaseDirs = project.projectDir.absolutePath
+        }
 
         logger.kotlinDebug("compiling with args ${ArgumentUtils.convertArgumentsToStringList(args)}")