Publish ContractsDsl

- Mark @ContractsDsl as @Experimental
- Move Contracts DSL out from 'internal' package
- Change visibility of ContractsDsl from 'internal' to 'public'

^KT-25274 Fixed
^KT-25495 Fixed

libraries/stdlib/common/build.gradle

@@ -54,6 +54,7 @@ compileKotlinCommon {
                 "-module-name", project.name,
                 "-Xuse-experimental=kotlin.Experimental",
                 "-Xuse-experimental=kotlin.ExperimentalUnsignedTypes",
+                "-Xuse-experimental=kotlin.contracts.ExperimentalContracts",
                 "-XXLanguage:+InlineClasses",
                 "-Xallow-kotlin-package"
         ]
@@ -66,6 +67,7 @@ compileCoroutinesKotlinCommon {
         apiVersion = "1.3"
         freeCompilerArgs = [
                 "-module-name", project.name+"-coroutines",
+                "-Xuse-experimental=kotlin.contracts.ExperimentalContracts",
                 "-Xallow-kotlin-package"
         ]
     }