Roll back the use of Object.assign() in evaluateSnippets.

This is required for fiddling with the global object on node 8.

lib/evaluateSnippets.js

@@ -26,6 +26,16 @@ try {
   };
 }
 
+function extend(target) {
+  for (var i = 1; i < arguments.length; i += 1) {
+    var source = arguments[i];
+    Object.keys(source).forEach(function(key) {
+      target[key] = source[key];
+    });
+  }
+  return target;
+}
+
 function isPromise(value) {
   return (
     value &&
@@ -55,7 +65,7 @@ function getErrorMessage(expect, format, error) {
 }
 
 module.exports = async function(snippets, options) {
-  var oldGlobal = Object.assign({}, global);
+  var oldGlobal = extend({}, global);
   var expect = createExpect(options);
   global.expect = expect.clone();
   global.require = require;
@@ -137,5 +147,6 @@ module.exports = async function(snippets, options) {
     }
   });
 
-  Object.assign(global, oldGlobal);
+  // local function used for compatibility with node 8
+  extend(global, oldGlobal);
 };