Fix PaperScript#compile() with prefix operators

Closes #1611

src/core/PaperScript.js

@@ -268,6 +268,11 @@ Base.exports.PaperScript = function() {
                                 str = exp;
                             str = arg + '; ' + str;
                         }
+                        // If this is a prefixed update expression (++a / --a),
+                        // wrap expression in IIFE to avoid several bugs (#1450)
+                        if (node.prefix) {
+                            str = '(function(){return '+str+';})()';
+                        }
                         replaceCode(node, str);
                     } else { // AssignmentExpression
                         if (/^.=$/.test(node.operator)

test/tests/PaperScript.js

@@ -0,0 +1,20 @@
+/*
+ * Paper.js - The Swiss Army Knife of Vector Graphics Scripting.
+ * http://paperjs.org/
+ *
+ * Copyright (c) 2011 - 2016, Juerg Lehni & Jonathan Puckey
+ * http://scratchdisk.com/ & https://puckey.studio/
+ *
+ * Distributed under the MIT license. See LICENSE file for details.
+ *
+ * All rights reserved.
+ */
+
+QUnit.module('PaperScript');
+
+test('PaperScript#compile() with prefix increment/decrement operators', function() {
+    var code = 'var x = 1; var y = 1 * --x;';
+    var compiled = PaperScript.compile(code, paper);
+    PaperScript.execute(compiled, paper);
+    expect(0);
+});

test/tests/load.js

@@ -63,6 +63,8 @@
 
 /*#*/ include('Numerical.js');
 
+/*#*/ include('PaperScript.js');
+
 // There is no need to test interactions in node context.
 if (!isNodeContext) {
     /*#*/ include('Interactions.js');