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/common/build.gradle

@@ -49,7 +49,8 @@ compileKotlinCommon {
 compileKotlinCommon {
     kotlinOptions {
         freeCompilerArgs = [
-                "-module-name", project.name
+                "-module-name", project.name,
+                "-Xuse-experimental=kotlin.Experimental"
         ]
     }
 }
@@ -94,4 +95,4 @@ task distCommon(type: Copy) {
 
 dist.dependsOn distCommon
 
-classes.setDependsOn(classes.dependsOn.findAll { it != "compileJava" })
+classes.setDependsOn(classes.dependsOn.findAll { it != "compileJava" })