avoid using .shift() in flush() (#4356)

src/runtime/internal/scheduler.ts

@@ -31,18 +31,23 @@ export function add_flush_callback(fn) {
 	flush_callbacks.push(fn);
 }
 
+let flushing = false;
 const seen_callbacks = new Set();
 export function flush() {
+	if (flushing) return;
+	flushing = true;
 
 	do {
 		// first, call beforeUpdate functions
 		// and update components
-		while (dirty_components.length) {
+		for (let i = 0; i < dirty_components.length; i += 1) {
-			const component = dirty_components.shift();
+			const component = dirty_components[i];
 			set_current_component(component);
 			update(component.$$);
 		}
 
+		dirty_components.length = 0;
+
 		while (binding_callbacks.length) binding_callbacks.pop()();
 
 		// then, once components are updated, call
@@ -67,6 +72,7 @@ export function flush() {
 	}
 
 	update_scheduled = false;
+	flushing = false;
 	seen_callbacks.clear();
 }