fix(orderBy): ensure the orderBy formatter can handle null values in collections

Author: heisaf

lib/formatter/order_by.dart

@@ -139,7 +139,13 @@ class OrderBy implements Function {
   static _nop(e) => e;
   static bool _isNonZero(int n) => (n != 0);
   static int _returnZero() => 0;
-  static int _defaultComparator(a, b) => Comparable.compare(a, b);
+  static int _defaultComparator(a, b) {
+    if (a == null && b == null) return 0;
+    if (a == null) return -1;
+    if (b == null) return 1;
+
+    return Comparable.compare(a, b);
+  }
   static int _reverseComparator(a, b) => _defaultComparator(b, a);
 
   static int _compareLists(List a, List b, List<Comparator> comparators) {

test/formatter/order_by_spec.dart

@@ -201,4 +201,45 @@ main() {
     });
 
   });
+
+  describe('null safe orderBy formatter', () {
+    var Emily___Bronte = new Name(firstName: 'Emily', lastName: 'Bronte'),
+        Mark____Twain = {'firstName': 'Mark',    'lastName': 'Twain'},
+        Jeffrey_Archer = {'firstName': 'Jeffrey', 'lastName': 'Archer'},
+        Isaac___Asimov = new Name(firstName: 'Isaac', lastName: 'Asimov'),
+        Oscar___Wilde = {'firstName': 'Oscar',   'lastName': 'Wilde'},
+        Oscar___Null = {'firstName': 'Oscar',   'lastName': null};
+    beforeEach((Scope scope, Parser parse, FormatterMap formatters) {
+      scope.context['authors'] = [
+          Emily___Bronte,
+          Mark____Twain,
+          Jeffrey_Archer,
+          Isaac___Asimov,
+          Oscar___Wilde,
+          Oscar___Null
+      ];
+    });
+    it('should allow null values',
+        (Scope scope, Parser parse, FormatterMap formatters) {
+          expect(parse('authors | orderBy:"lastName"').eval(scope.context, formatters)).toEqual([
+              Oscar___Null,
+              Jeffrey_Archer,
+              Isaac___Asimov,
+              Emily___Bronte,
+              Mark____Twain,
+              Oscar___Wilde,
+          ]);
+    });
+    it('should allow null values in reverse order',
+        (Scope scope, Parser parse, FormatterMap formatters) {
+      expect(parse('authors | orderBy:"-lastName"').eval(scope.context, formatters)).toEqual([
+          Oscar___Wilde,
+          Mark____Twain,
+          Emily___Bronte,
+          Isaac___Asimov,
+          Jeffrey_Archer,
+          Oscar___Null,
+      ]);
+    });
+  });
 }