Introduce OptionalExpectation for annotations missing on some platforms

This commits adds a new annotation OptionalExpectation to the standard
library, which is experimental. To enable its usage, either pass
'-Xuse-experimental=kotlin.ExperimentalMultiplatform' as a compiler
argument, or '-Xuse-experimental=kotlin.Experimental' and also annotate
each usage with `@UseExperimental(ExperimentalMultiplatform::class)`

 #KT-18882 Fixed

libraries/stdlib/js/build.gradle

@@ -137,7 +137,10 @@ compileKotlin2Js {
         outputFile = "${buildDir}/classes/main/kotlin.js"
         sourceMap = true
         sourceMapPrefix = "./"
-        freeCompilerArgs += ["-source-map-base-dirs", [builtinsSrcDir, jsSrcDir, commonSrcDir, commonSrcDir2].collect { file(it).absoluteFile }.join(File.pathSeparator)]
+        freeCompilerArgs += [
+            "-source-map-base-dirs", [builtinsSrcDir, jsSrcDir, commonSrcDir, commonSrcDir2].collect { file(it).absoluteFile }.join(File.pathSeparator),
+            "-Xuse-experimental=kotlin.Experimental"
+        ]
     }
 }