Add EXACTLY_ONCE contract to functions that call their lambda parameter once

KT-35972

libraries/stdlib/js/src/kotlinx/dom/Builders.kt

@@ -6,19 +6,25 @@
 package kotlinx.dom
 
 import org.w3c.dom.*
+import kotlin.contracts.*
 
 /**
  * Creates a new element with the specified [name].
  *
  * The element is initialized with the specified [init] function.
  */
-public fun Document.createElement(name: String, init: Element.() -> Unit): Element = createElement(name).apply(init)
+public fun Document.createElement(name: String, init: Element.() -> Unit): Element {
+    contract { callsInPlace(init, InvocationKind.EXACTLY_ONCE) }
+    return createElement(name).apply(init)
+}
 
 /**
  * Appends a newly created element with the specified [name] to this element.
  *
  * The element is initialized with the specified [init] function.
  */
-public fun Element.appendElement(name: String, init: Element.() -> Unit): Element =
-    ownerDocument!!.createElement(name, init).also { appendChild(it) }
+public fun Element.appendElement(name: String, init: Element.() -> Unit): Element {
+    contract { callsInPlace(init, InvocationKind.EXACTLY_ONCE) }
+    return ownerDocument!!.createElement(name, init).also { appendChild(it) }
+}