[js] html externs: add Null<T> to return types where appropriate

std/js/html/BodyElement.hx

@@ -66,7 +66,7 @@ extern class BodyElement extends Element
 	var background : String;
 	var onafterprint : haxe.Constraints.Function;
 	var onbeforeprint : haxe.Constraints.Function;
-	var onbeforeunload : Event -> String;
+	var onbeforeunload : Event -> Null<String>;
 	var onhashchange : haxe.Constraints.Function;
 	var onlanguagechange : haxe.Constraints.Function;
 	var onmessage : haxe.Constraints.Function;

std/js/html/Document.hx

@@ -543,11 +543,11 @@ extern class Document extends Node
 	/** @throws DOMError */
 	@:overload( function( expression : String, ?resolver : haxe.Constraints.Function) : XPathExpression {} )
 	@:overload( function( expression : String, ?resolver : XPathNSResolver) : XPathExpression {} )
-	function createExpression( expression : String, ?resolver : String -> String ) : XPathExpression;
+	function createExpression( expression : String, ?resolver : String -> Null<String> ) : XPathExpression;
 	@:pure
 	function createNSResolver( nodeResolver : Node ) : Node;
 	/** @throws DOMError */
 	@:overload( function( expression : String, contextNode : Node, ?resolver : haxe.Constraints.Function, ?type : Int = 0, ?result : Dynamic) : XPathResult {} )
 	@:overload( function( expression : String, contextNode : Node, ?resolver : XPathNSResolver, ?type : Int = 0, ?result : Dynamic) : XPathResult {} )
-	function evaluate( expression : String, contextNode : Node, ?resolver : String -> String, ?type : Int = 0, ?result : Dynamic ) : XPathResult;
+	function evaluate( expression : String, contextNode : Node, ?resolver : String -> Null<String>, ?type : Int = 0, ?result : Dynamic ) : XPathResult;
 }

std/js/html/FrameSetElement.hx

@@ -46,7 +46,7 @@ extern class FrameSetElement extends Element
 	var rows : String;
 	var onafterprint : haxe.Constraints.Function;
 	var onbeforeprint : haxe.Constraints.Function;
-	var onbeforeunload : Event -> String;
+	var onbeforeunload : Event -> Null<String>;
 	var onhashchange : haxe.Constraints.Function;
 	var onlanguagechange : haxe.Constraints.Function;
 	var onmessage : haxe.Constraints.Function;

std/js/html/Window.hx

@@ -344,7 +344,7 @@ extern class Window extends EventTarget
 	var ontouchcancel : haxe.Constraints.Function;
 	var onafterprint : haxe.Constraints.Function;
 	var onbeforeprint : haxe.Constraints.Function;
-	var onbeforeunload : Event -> String;
+	var onbeforeunload : Event -> Null<String>;
 	var onhashchange : haxe.Constraints.Function;
 	var onlanguagechange : haxe.Constraints.Function;
 	var onmessage : haxe.Constraints.Function;

std/js/html/XPathEvaluator.hx

@@ -32,11 +32,11 @@ extern class XPathEvaluator
 	/** @throws DOMError */
 	@:overload( function( expression : String, ?resolver : haxe.Constraints.Function) : XPathExpression {} )
 	@:overload( function( expression : String, ?resolver : XPathNSResolver) : XPathExpression {} )
-	function createExpression( expression : String, ?resolver : String -> String ) : XPathExpression;
+	function createExpression( expression : String, ?resolver : String -> Null<String> ) : XPathExpression;
 	@:pure
 	function createNSResolver( nodeResolver : Node ) : Node;
 	/** @throws DOMError */
 	@:overload( function( expression : String, contextNode : Node, ?resolver : haxe.Constraints.Function, ?type : Int = 0, ?result : Dynamic) : XPathResult {} )
 	@:overload( function( expression : String, contextNode : Node, ?resolver : XPathNSResolver, ?type : Int = 0, ?result : Dynamic) : XPathResult {} )
-	function evaluate( expression : String, contextNode : Node, ?resolver : String -> String, ?type : Int = 0, ?result : Dynamic ) : XPathResult;
+	function evaluate( expression : String, contextNode : Node, ?resolver : String -> Null<String>, ?type : Int = 0, ?result : Dynamic ) : XPathResult;
 }