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

See KT-19818

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/Kotlin2JsGradlePluginIT.kt

@@ -195,7 +195,7 @@ class Kotlin2JsGradlePluginIT : BaseGradleIT() {
             assertFileExists(mapFilePath)
             val map = fileInWorkingDir(mapFilePath).readText()
 
-            val sourceFilePath = "prefixprefix/example/Dummy.kt"
+            val sourceFilePath = "prefixprefix/src/main/kotlin/example/Dummy.kt"
             assertTrue("Source map should contain reference to $sourceFilePath") { map.contains("\"$sourceFilePath\"") }
         }
     }
@@ -211,8 +211,8 @@ class Kotlin2JsGradlePluginIT : BaseGradleIT() {
             assertFileExists(mapFilePath)
             val map = fileInWorkingDir(mapFilePath).readText()
 
-            assertTrue("Source map should contain reference to main.kt") { map.contains("\"main.kt\"") }
-            assertTrue("Source map should contain reference to foo.kt") { map.contains("\"foo.kt\"") }
+            assertTrue("Source map should contain reference to main.kt") { map.contains("\"./src/main/kotlin/main.kt\"") }
+            assertTrue("Source map should contain reference to foo.kt") { map.contains("\"./src/main/kotlin/foo.kt\"") }
             assertTrue("Source map should contain source of main.kt") { map.contains("\"fun main(args: Array<String>) {\\n") }
             assertTrue("Source map should contain source of foo.kt") { map.contains("\"inline fun foo(): String {\\n") }
         }

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlin2JsProjectWithSourceMapInline/build.gradle

@@ -24,6 +24,7 @@ allprojects {
     compileKotlin2Js {
         kotlinOptions.freeCompilerArgs = [ "-Xskip-metadata-version-check" ]
         kotlinOptions.sourceMap = true
+        kotlinOptions.sourceMapPrefix = "./"
         kotlinOptions.sourceMapEmbedSources = "always"
     }
 }

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)}")