fix(select): Corrected NPE if select multiple nested in ng-if

Closes #428

lib/directive/input_select.dart

@@ -220,7 +220,14 @@ class _MultipleSelectionMode extends _SelectMode {
     Function fn = (o, i) => o.selected = null;
 
     if (selectedValues is List) {
-      fn = (o, i) => o.selected = selectedValues.contains(expando[o].ngValue);
+      fn = (o, i) {
+        var selected = expando[o];
+        if (selected == null) {
+          return false;
+        } else {
+          return o.selected = selectedValues.contains(selected.ngValue);
+        }
+      };
     }
 
     _forEachOption(fn);

test/directive/input_select_spec.dart

@@ -398,6 +398,29 @@ main() {
         _.rootScope.$apply();
         expect(_.rootElement).toEqualSelect([['a'], 'b']);
       });
+
+
+      it('issue #428', () {
+        _.compile(
+            '<div>' +
+              '<div ng-if="attached">' +
+                '<select ng-model="model" multiple>' +
+                  '<option value="a">foo</option>' +
+                  '<option value="b">bar</option>' +
+                '</select>' +
+              '</div>' +
+            '</div>');
+        _.rootScope.model = ['a'];
+        _.rootScope.attached = true;
+        _.rootScope.$apply();
+        expect(_.rootElement).toEqualSelect([['a'], 'b']);
+        _.rootScope.attached = false;
+        _.rootScope.$apply();
+        expect(_.rootElement).toEqualSelect([]);
+        _.rootScope.attached = true;
+        _.rootScope.$apply();
+        expect(_.rootElement).toEqualSelect([['a'], 'b']);
+      });
     });