Revert "Temporarily remove isInitialized and tests, but keep the implementation"

This reverts commit 234148518e.

libraries/stdlib/src/kotlin/internal/Annotations.kt

@@ -66,6 +66,15 @@ internal annotation class InlineOnly
 @Retention(AnnotationRetention.BINARY)
 internal annotation class DynamicExtension
 
+/**
+ * The value of this parameter should be a property reference expression (`this::foo`), referencing a `lateinit` property,
+ * the backing field of which is accessible at the point where the corresponding argument is passed.
+ */
+@Target(AnnotationTarget.VALUE_PARAMETER)
+@Retention(AnnotationRetention.BINARY)
+@SinceKotlin("1.2")
+internal annotation class AccessibleLateinitPropertyLiteral
+
 /**
  * Specifies that this declaration is only completely supported since the specified version.
  *

libraries/stdlib/src/kotlin/util/Lateinit.kt

@@ -0,0 +1,19 @@
+@file:kotlin.jvm.JvmName("LateinitKt")
+@file:kotlin.jvm.JvmVersion
+@file:Suppress("unused")
+
+package kotlin
+
+import kotlin.internal.InlineOnly
+import kotlin.internal.AccessibleLateinitPropertyLiteral
+import kotlin.reflect.KProperty0
+
+/**
+ * Returns `true` if this lateinit property has been assigned a value, and `false` otherwise.
+ *
+ * Cannot be used in an inline function, to avoid binary compatibility issues.
+ */
+@SinceKotlin("1.2")
+@InlineOnly
+inline val @receiver:AccessibleLateinitPropertyLiteral KProperty0<*>.isInitialized: Boolean
+    get() = throw NotImplementedError("Implementation is intrinsic")