Avoid _.extend in IE8-sensitive ecmascript tests.

The underlying problem is that `es5-shim` polyfills `Array.prototype`
methods in IE8, but it can't actually make them non-enumerable, so
`_.extend` copies them, because it doesn't check `hasOwnProperty`.

This probably calls for a more general audit of for-in loops over objects
that could be Arrays, especially within underscore.

packages/ecmascript/runtime-tests.js

@@ -1,7 +1,10 @@
 Tinytest.add("ecmascript - runtime - template literals", (test) => {
   function dump(pieces) {
-    return [_.extend({}, pieces),
-            _.toArray(arguments).slice(1)];
+    var copy = {};
+    // Can't use _.extend({}, pieces) because es5-shim adds enumerable
+    // methods to Array.prototype, and _.extend has no own property check.
+    _.each(_.keys(pieces), key => copy[key] = pieces[key]);
+    return [copy, _.toArray(arguments).slice(1)];
   };
   const foo = 'B';
   // uses `babelHelpers.taggedTemplateLiteralLoose`