fix(DateFilter): fix a wrong type

Closes #579

lib/filter/date.dart

@@ -21,8 +21,8 @@ part of angular.filter;
  *
  */
 @NgFilter(name:'date')
-class DateFilter {
-  static Map<String, String> MAP = {
+class DateFilter implements Function {
+  static final _MAP = const <String, String> {
     'medium':     'MMM d, y h:mm:ss a',
     'short':      'M/d/yy h:mm a',
     'fullDate':   'EEEE, MMMM d, y',
@@ -33,7 +33,7 @@ class DateFilter {
     'shortTime':  'h:mm a',
   };
 
-  Map<num, NumberFormat> nfs = new Map<num, NumberFormat>();
+  var _dfs = <String, DateFormat>{};
 
   /**
    *  [date]: Date to format either as Date object, milliseconds
@@ -47,18 +47,19 @@ class DateFilter {
    *    mediumDate is used
    *
    */
-  call(date, [format = r'mediumDate']) {
+  dynamic call(Object date, [String format = 'mediumDate']) {
     if (date == '' || date == null) return date;
     if (date is String) date = DateTime.parse(date);
     if (date is num) date = new DateTime.fromMillisecondsSinceEpoch(date);
     if (date is! DateTime) return date;
-    var nf = nfs[format];
-    if (nf == null) {
-      if (MAP.containsKey(format)) {
-        format = MAP[format];
+    var df = _dfs[format];
+    if (df == null) {
+      if (_MAP.containsKey(format)) {
+        format = _MAP[format];
       }
-      nf = new DateFormat(format);
+      df = new DateFormat(format);
+      _dfs[format] = df;
     }
-    return nf.format(date);
+    return df.format(date);
   }
 }

test/filter/date_spec.dart

@@ -3,9 +3,9 @@ library date_spec;
 import '../_specs.dart';
 
 main() => describe('date', () {
-  var morning  = DateTime.parse('2010-09-03T07:05:08.008Z'); //7am
-  var noon =     DateTime.parse('2010-09-03T12:05:08.012Z'); //12pm
-  var midnight = DateTime.parse('2010-09-03T12:05:08.123Z'); //12am
+  var morning   = DateTime.parse('2010-09-03T07:05:08.008Z'); //7am
+  var noon      = DateTime.parse('2010-09-03T12:05:08.012Z'); //12pm
+  var midnight  = DateTime.parse('2010-09-03T12:05:08.123Z'); //12am
   var earlyDate = DateTime.parse('0001-09-03T05:05:08.000Z');
 
   var date;
@@ -57,4 +57,10 @@ main() => describe('date', () {
     expect(date(noon, "shortTime")).
     toEqual('12:05 PM');
   });
+
+  it('should use cache without any error', () {
+
+    date(noon, "shortTime");
+    date(noon, "shortTime");
+  });
 });