add enumerate to the default filter set

pkg/polymer_expressions/lib/polymer_expressions.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * A binding delegate used with Polymer elements that
  * allows for complex binding expressions, including
  * property access, function invocation,
  * list/map indexing, and two-way filtering.
  *
@@ skipping 18 matching lines @@
 
 import 'dart:async';
 import 'dart:html';
 
 import 'package:observe/observe.dart';
 import 'package:logging/logging.dart';
 
 import 'eval.dart';
 import 'expression.dart';
 import 'parser.dart';
+import 'src/globals.dart';
 
 final Logger _logger = new Logger('polymer_expressions');
 
 // TODO(justin): Investigate XSS protection
 Object _classAttributeConverter(v) =>
     (v is Map) ? v.keys.where((k) => v[k] == true).join(' ') :
     (v is Iterable) ? v.join(' ') :
     v;
 
 Object _styleAttributeConverter(v) =>
     (v is Map) ? v.keys.map((k) => '$k: ${v[k]}').join(';') :
     (v is Iterable) ? v.join(';') :
     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;
 
   _Binding getBinding(model, String path, name, node) {
     if (path == null) return null;
     var expr = new Parser(path).parse();
     if (model is! Scope) {
       model = new Scope(model: model, variables: globals);
     }
     if (node is Element && name == "class") {
       return new _Binding(expr, model, _classAttributeConverter);
     }
@@ skipping 61 matching lines @@
     }
   }
 
   getValueWorkaround(key) {
     if (key == _VALUE) return value;
   }
 
   setValueWorkaround(key, v) {
     if (key == _VALUE) value = v;
   }
-
 }