Install psalm as dev, sync scripts updates

vendor/amphp/amp/lib/LazyPromise.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace Amp;
+
+/**
+ * Creates a promise that calls $promisor only when the result of the promise is requested (i.e. onResolve() is called
+ * on the promise). $promisor can return a promise or any value. If $promisor throws an exception, the promise fails
+ * with that exception. If $promisor returns a Generator, it will be run as a coroutine.
+ */
+final class LazyPromise implements Promise
+{
+    /** @var callable|null */
+    private $promisor;
+
+    /** @var Promise|null */
+    private $promise;
+
+    /**
+     * @param callable $promisor Function which starts an async operation, returning a Promise (or any value).
+     *     Generators will be run as a coroutine.
+     */
+    public function __construct(callable $promisor)
+    {
+        $this->promisor = $promisor;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function onResolve(callable $onResolved)
+    {
+        if ($this->promise === null) {
+            \assert($this->promisor !== null);
+
+            $provider = $this->promisor;
+            $this->promisor = null;
+            $this->promise = call($provider);
+        }
+
+        \assert($this->promise !== null);
+
+        $this->promise->onResolve($onResolved);
+    }
+}