Make mappedBy lazy.

tests/html/xmldocument_test.dart

Index: tests/html/xmldocument_test.dart
diff --git a/tests/html/xmldocument_test.dart b/tests/html/xmldocument_test.dart
index c5e672fa6bf9e20cb2fea7488d09af268bc92304..d9afa92a665e062eb0e7af5e8b7f676eba25c108 100644
--- a/tests/html/xmldocument_test.dart
+++ b/tests/html/xmldocument_test.dart
@@ -41,7 +41,8 @@ main() {
   group('elements', () {
     test('filters out non-element nodes', () {
       final doc = new XMLDocument.xml("<xml>1<a/><b/>2<c/>3<d/></xml>");
-      expect(doc.elements.mappedBy((e) => e.tagName), ["a", "b", "c", "d"]);
+      expect(doc.elements.mappedBy((e) => e.tagName).toList(),
+             ["a", "b", "c", "d"]);
     });
 
     test('overwrites nodes when set', () {
@@ -102,7 +103,7 @@ main() {
     });
 
     test('map', () {
-      expect(makeClassSet().mappedBy((c) => c.toUpperCase()),
+      expect(makeClassSet().mappedBy((c) => c.toUpperCase()).toList(),
           unorderedEquals(['FOO', 'BAR', 'BAZ']));
     });
 
@@ -427,7 +428,7 @@ main() {
   test('queryAll', () {
     final doc = new XMLDocument.xml(
         "<xml><foo id='f1' /><bar><foo id='f2' /></bar></xml>");
-    expect(doc.queryAll('foo').mappedBy((e) => e.id), ['f1', 'f2']);
+    expect(doc.queryAll('foo').mappedBy((e) => e.id).toList(), ['f1', 'f2']);
     expect(doc.queryAll('baz'), []);
   });