Forbid "introduce backing property" for expect classes #KT-28382 Fixed

idea/testData/intentions/introduceBackingProperty/expectClass.kt

@@ -0,0 +1,8 @@
+// IS_APPLICABLE: false
+// ERROR: The feature "multi platform projects" is experimental and should be enabled explicitly
+
+expect class Outer {
+    class Nested {
+        val <caret>x: Int
+    }
+}