Support partial parsing of $foo->

Introduce Error node for this purpose.

grammar/php5.y

@@ -915,6 +915,7 @@ object_property:
       T_STRING                                              { $$ = $1; }
     | '{' expr '}'                                          { $$ = $2; }
     | variable_without_objects                              { $$ = $1; }
+    | error                                                 { $$ = Expr\Error[]; }
 ;
 
 list_expr:

grammar/php7.y

@@ -793,6 +793,7 @@ property_name:
       T_STRING                                              { $$ = $1; }
     | '{' expr '}'	                                        { $$ = $2; }
     | simple_variable	                                    { $$ = Expr\Variable[$1]; }
+    | error                                                 { $$ = Expr\Error[]; }
 ;
 
 list_expr:

lib/PhpParser/Node/Expr/Error.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace PhpParser\Node\Expr;
+
+use PhpParser\Node\Expr;
+
+/**
+ * Error node used during parsing with error recovery.
+ *
+ * An error node may be placed at a position where an expression is required, but an error occurred.
+ * Error nodes will not be present if the parser is run in throwOnError mode (the default).
+ */
+class Error extends Expr
+{
+    /**
+     * Constructs an error node.
+     *
+     * @param array $attributes Additional attributes
+     */
+    public function __construct(array $attributes = array()) {
+        parent::__construct($attributes);
+    }
+
+    public function getSubNodeNames() {
+        return array();
+    }
+}