Implement DurationUnit in JS, add doc summary and conversion test

libraries/stdlib/jvm/src/kotlin/time/DurationUnitJvm.kt

@@ -1,6 +1,6 @@
 /*
- * Copyright 2010-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
- * that can be found in the license/LICENSE.txt file.
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
 @file:kotlin.jvm.JvmMultifileClass()
@@ -8,14 +8,8 @@
 
 package kotlin.time
 
-//Actual typealias 'DurationUnit' has no corresponding expected declaration
-//The following declaration is incompatible because modality is different:
-//    public final expect enum class DurationUnit : Enum<DurationUnit>
-
-@Suppress("ACTUAL_WITHOUT_EXPECT")
 public actual typealias DurationUnit = java.util.concurrent.TimeUnit
 
-
 public actual fun convertDurationUnit(value: Double, sourceUnit: DurationUnit, targetUnit: DurationUnit): Double {
     val sourceInTargets = targetUnit.convert(1, sourceUnit)
     if (sourceInTargets > 0)