KT-27856 Add contracts to Timing.kt lambdas (#1987)

libraries/stdlib/jvm/src/kotlin/system/Timing.kt

@@ -6,10 +6,15 @@
 @file:kotlin.jvm.JvmName("TimingKt")
 package kotlin.system
 
+import kotlin.contracts.*
+
 /**
  * Executes the given [block] and returns elapsed time in milliseconds.
  */
 public inline fun measureTimeMillis(block: () -> Unit): Long {
+    contract {
+        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+    }
     val start = System.currentTimeMillis()
     block()
     return System.currentTimeMillis() - start
@@ -19,6 +24,9 @@ public inline fun measureTimeMillis(block: () -> Unit): Long {
  * Executes the given [block] and returns elapsed time in nanoseconds.
  */
 public inline fun measureNanoTime(block: () -> Unit): Long {
+    contract {
+        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+    }
     val start = System.nanoTime()
     block()
     return System.nanoTime() - start