Move fragment type check to normalizer (#2462)

Should result in more informative stack traces.

Fixes #2461

render/vnode.js

@@ -11,8 +11,19 @@ Vnode.normalize = function(node) {
 }
 Vnode.normalizeChildren = function(input) {
 	var children = []
-	for (var i = 0; i < input.length; i++) {
-		children[i] = Vnode.normalize(input[i])
+	if (input.length) {
+		var isKeyed = input[0] != null && input[0].key != null
+		// Note: this is a *very* perf-sensitive check.
+		// Fun fact: merging the loop like this is somehow faster than splitting
+		// it, noticeably so.
+		for (var i = 1; i < input.length; i++) {
+			if ((input[i] != null && input[i].key != null) !== isKeyed) {
+				throw new TypeError("Vnodes must either always have keys or never have keys!")
+			}
+		}
+		for (var i = 0; i < input.length; i++) {
+			children[i] = Vnode.normalize(input[i])
+		}
 	}
 	return children
 }