Make mappedBy lazy.

pkg/intl/test/date_time_format_http_request_test.dart

Index: pkg/intl/test/date_time_format_http_request_test.dart
diff --git a/pkg/intl/test/date_time_format_http_request_test.dart b/pkg/intl/test/date_time_format_http_request_test.dart
index d11a0deefe2e7063e2655dac838f02c7b0bb91f5..2fb5d9914288ccfc210a2813f810391fd1a21cad 100644
--- a/pkg/intl/test/date_time_format_http_request_test.dart
+++ b/pkg/intl/test/date_time_format_http_request_test.dart
@@ -25,8 +25,9 @@ main() {
 
 void runEverything(_) {
   // Initialize all locales and wait for them to finish before running tests.
-  var futures = DateFormat.allLocalesWithSymbols().mappedBy(
-      (locale) => initializeDateFormatting(locale, url));
+  var futures = DateFormat.allLocalesWithSymbols()
+      .mappedBy((locale) => initializeDateFormatting(locale, url))
+      .toList();
   Futures.wait(futures).then(expectAsync1((_) {
       runDateTests(smallSetOfLocales());
       shutDown();}));