fix unit tests under recent HHVM

Author: fredemmott

src/Reflection/ReflectionXHPAttribute.hack

@@ -91,11 +91,11 @@ class ReflectionXHPAttribute {
     );
     $v = $this->extraType;
     invariant(
-      \HH\is_php_array($v),
+      $v is Container<_>,
       'Class name for attribute %s is not an array',
       $this->getName(),
     );
-    return keyset($v);
+    return keyset(/* HH_FIXME[4110] not limited to arraykey */ $v);
   }
 
   /**

src/Reflection/ReflectionXHPChildrenDeclaration.hack

@@ -42,7 +42,7 @@ class ReflectionXHPChildrenDeclaration {
 
   <<__Memoize>>
   public function getType(): XHPChildrenDeclarationType {
-    if (\HH\is_php_array($this->data)) {
+    if ($this->data is Container<_>) {
       return XHPChildrenDeclarationType::EXPRESSION;
     }
     return XHPChildrenDeclarationType::assert($this->data);