dart2js implementation of StringBuffer.writeAll that optimizes better

sdk/lib/_internal/pub/test/dart2js/source_maps_include_core_libs_in_subdirectory_test.dart

Index: sdk/lib/_internal/pub/test/dart2js/source_maps_include_core_libs_in_subdirectory_test.dart
diff --git a/sdk/lib/_internal/pub/test/dart2js/source_maps_include_core_libs_in_subdirectory_test.dart b/sdk/lib/_internal/pub/test/dart2js/source_maps_include_core_libs_in_subdirectory_test.dart
index b48c99e502babeeeed093a8326e8a1c7185d0b3f..cedb073b9870fe51a42056de7ce41fd63ab8fc3c 100644
--- a/sdk/lib/_internal/pub/test/dart2js/source_maps_include_core_libs_in_subdirectory_test.dart
+++ b/sdk/lib/_internal/pub/test/dart2js/source_maps_include_core_libs_in_subdirectory_test.dart
@@ -15,14 +15,14 @@ main() {
   // This test is a bit shaky. Since dart2js is free to inline things, it's
   // not precise as to which source libraries will actually be referenced in
   // the source map. But this tries to use a type in the core library
-  // (StringBuffer) and validate that its source ends up in the source map.
+  // (Duration) and validate that its source ends up in the source map.
   integration("Dart core libraries are available to source maps when the "
       "build directory is a subdirectory", () {
     d.dir(appPath, [
       d.appPubspec(),
       d.dir("web", [
         d.dir("sub", [
-          d.file("main.dart", "main() => new StringBuffer().writeAll(['s']);")
+          d.file("main.dart", "main() => new Duration().toString();")
         ])
       ])
     ]).create();
@@ -31,10 +31,10 @@ main() {
     pubServe(args: [webSub]);
 
     requestShouldSucceed("main.dart.js.map",
-        contains(r"packages/$sdk/lib/core/string_buffer.dart"),
+        contains(r"packages/$sdk/lib/core/duration.dart"),
         root: webSub);
-    requestShouldSucceed(r"packages/$sdk/lib/core/string_buffer.dart",
-        contains("class StringBuffer"),
+    requestShouldSucceed(r"packages/$sdk/lib/core/duration.dart",
+        contains("class Duration"),
         root: webSub);
 
     endPubServe();