Make mappedBy lazy.

tests/html/xmlelement_test.dart

Index: tests/html/xmlelement_test.dart
diff --git a/tests/html/xmlelement_test.dart b/tests/html/xmlelement_test.dart
index 5c20d44ec05c65f533f50406aa8d125f2ac250e0..4f9c1f9d398db2b9a1543e13223c59101f9c1443 100644
--- a/tests/html/xmlelement_test.dart
+++ b/tests/html/xmlelement_test.dart
@@ -57,7 +57,8 @@ main() {
   group('elements', () {
     test('filters out non-element nodes', () {
       final el = new XMLElement.xml("<xml>1<a/><b/>2<c/>3<d/></xml>");
-      expect(el.elements.mappedBy((e) => e.tagName), ["a", "b", "c", "d"]);
+      expect(el.elements.mappedBy((e) => e.tagName).toList(),
+             ["a", "b", "c", "d"]);
     });
 
     test('overwrites nodes when set', () {
@@ -115,7 +116,7 @@ main() {
     });
 
     test('map', () {
-      expect(makeClassSet().mappedBy((c) => c.toUpperCase()),
+      expect(makeClassSet().mappedBy((c) => c.toUpperCase()).toList(),
           unorderedEquals(['FOO', 'BAR', 'BAZ']));
     });
 
@@ -417,7 +418,7 @@ main() {
   test('queryAll', () {
     final el = new XMLElement.xml(
         "<xml><foo id='f1' /><bar><foo id='f2' /></bar></xml>");
-    expect(el.queryAll('foo').mappedBy((e) => e.id), ['f1', 'f2']);
+    expect(el.queryAll('foo').mappedBy((e) => e.id).toList(), ['f1', 'f2']);
     expect(el.queryAll('baz'), []);
   });