add enumerate to the default filter set

pkg/polymer_expressions/lib/polymer_expressions.dart

Index: pkg/polymer_expressions/lib/polymer_expressions.dart
diff --git a/pkg/polymer_expressions/lib/polymer_expressions.dart b/pkg/polymer_expressions/lib/polymer_expressions.dart
index dd7d7af922ff6993199749f82ea2cea972f62a2c..e94e3337a407c593f530dd1604220a1e90b49664 100644
--- a/pkg/polymer_expressions/lib/polymer_expressions.dart
+++ b/pkg/polymer_expressions/lib/polymer_expressions.dart
@@ -12,6 +12,7 @@ import 'package:observe/observe.dart';
 import 'eval.dart';
 import 'expression.dart';
 import 'parser.dart';
+import 'src/globals.dart';
 
 // TODO(justin): Investigate XSS protection
 Object _classAttributeConverter(v) =>
@@ -25,11 +26,19 @@ Object _styleAttributeConverter(v) =>
     v;
 
 class PolymerExpressions extends BindingDelegate {
+  /** The default [globals] to use for Polymer expressions. */
+  static const Map DEFAULT_GLOBALS = const { 'enumerate': enumerate };
 
   final Map<String, Object> globals;
 
+  /**
+   * Creates a new binding delegate for Polymer expressions, with the provided
+   * variables used as [globals]. If no globals are supplied, a copy of the
+   * [DEFAULT_GLOBALS] will be used.
+   */
   PolymerExpressions({Map<String, Object> globals})
-      : globals = (globals == null) ? new Map<String, Object>() : globals;
+      : globals = (globals == null) ?
+          new Map<String, Object>.from(DEFAULT_GLOBALS) : globals;

[justinfagnani, 2013/09/17 21:45:05]

I think I'd rather see the default globals set in PolymerElement, but I'm fine
either way.

[Jennifer Messerly, 2013/10/08 00:55:01]

yeah, I was kinda wondering that too. So far we don't have any syntax knowledge
in the Polymer library other than saying "new PolymerExpressions", so that's why
I'm leaning this way.

Definitely something we should bring up tomorrow when chatting with them. It
feels like there isn't an easy way to add a filter to the scope. The JS approach
of making it global doesn't seem good.

maybe if we had something like:

import 'package:quiver/quiver.dart' show enumerate;

class FancyButton extends PolymerElement {
  get templateGlobals => { 'enumerate': enumerate };
}

... would that be lightweight enough, I wonder?

 
   _Binding getBinding(model, String path, name, node) {
     if (path == null) return null;
@@ -105,5 +114,4 @@ class _Binding extends Object with ChangeNotifierMixin {
   setValueWorkaround(key, v) {
     if (key == _VALUE) value = v;
   }
-
 }