Implement DurationUnit in JS, add doc summary and conversion test

libraries/stdlib/test/time/DurationUnitTest.kt

@@ -0,0 +1,46 @@
+/*
+ * 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.
+ */
+
+package test.time
+
+import kotlin.test.*
+import kotlin.time.*
+// TODO: Use star import after KT-30983 is fixed
+import kotlin.time.DurationUnit.DAYS
+import kotlin.time.DurationUnit.HOURS
+import kotlin.time.DurationUnit.MINUTES
+import kotlin.time.DurationUnit.SECONDS
+import kotlin.time.DurationUnit.MILLISECONDS
+import kotlin.time.DurationUnit.MICROSECONDS
+import kotlin.time.DurationUnit.NANOSECONDS
+
+class DurationUnitTest {
+
+    @Test
+    fun conversion() {
+        fun test(sourceValue: Double, sourceUnit: DurationUnit, targetValue: Double, targetUnit: DurationUnit) {
+            assertEquals(
+                targetValue, convertDurationUnit(sourceValue, sourceUnit, targetUnit),
+                "Expected $sourceValue $sourceUnit to be $targetValue $targetUnit"
+            )
+            assertEquals(
+                sourceValue, convertDurationUnit(targetValue, targetUnit, sourceUnit),
+                "Expected $targetValue $targetUnit to be $sourceValue $sourceUnit"
+            )
+        }
+        test(1.0, MINUTES, 60.0, SECONDS)
+        test(30.0, MINUTES, 0.5, HOURS)
+        test(12.0, HOURS, 0.5, DAYS)
+        test(720.0, MINUTES, 0.5, DAYS)
+        test(1.0, DAYS, 86400.0, SECONDS)
+        test(1.0, DAYS, 86400e9, NANOSECONDS)
+        test(50.0, NANOSECONDS, 0.05, MICROSECONDS)
+        test(50.0, NANOSECONDS, 50e-9, SECONDS)
+        test(16.0, MILLISECONDS, 0.016, SECONDS)
+    }
+
+
+
+}