Implement MonoClock in JS

It uses either process.hrtime() in node.js or performance.now() in browser,
falling back to Date.now() if the latter is not available

libraries/stdlib/test/time/MeasureTimeTest.kt

@@ -0,0 +1,47 @@
+/*
+ * 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.random.Random
+import kotlin.test.*
+import kotlin.time.*
+
+class MeasureTimeTest {
+
+    private fun longRunningCalc(): String = buildString {
+        repeat(10) {
+            while (Random.nextDouble() >= 0.001);
+            append(('a'..'z').random())
+        }
+    }
+
+    @Test
+    fun measureTimeOfCalc() {
+        val someResult: String
+
+        val elapsed = measureTime {
+            someResult = longRunningCalc()
+        }
+
+        println("elapsed: $elapsed")
+
+        assertEquals(10, someResult.length)
+        assertTrue(elapsed > Duration.ZERO)
+    }
+
+    @Test
+    fun measureTimeAndResult() {
+        val someResult: String
+
+        val measured: DurationMeasured<String> = withMeasureTime { longRunningCalc().also { someResult = it } }
+        println("measured: $measured")
+
+        val (result, elapsed) = measured
+
+        assertEquals(someResult, result)
+        assertTrue(elapsed > Duration.ZERO)
+    }
+}