dart2js implementation of StringBuffer.writeAll that optimizes better

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

Index: sdk/lib/_internal/pub/test/dart2js/source_maps_include_core_libs_test.dart
diff --git a/sdk/lib/_internal/pub/test/dart2js/source_maps_include_core_libs_test.dart b/sdk/lib/_internal/pub/test/dart2js/source_maps_include_core_libs_test.dart
index b8efddd9a7f4c293232404b20a62b48a538f9642..6af59c6512c838a23102945b947cebd972d01fec 100644
--- a/sdk/lib/_internal/pub/test/dart2js/source_maps_include_core_libs_test.dart
+++ b/sdk/lib/_internal/pub/test/dart2js/source_maps_include_core_libs_test.dart
@@ -13,14 +13,14 @@ 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", () {
     d.dir(appPath, [
       d.appPubspec(),
       d.dir("web", [
-        d.file("main.dart", "main() => new StringBuffer().writeAll(['s']);"),
+        d.file("main.dart", "main() => new Duration().toString();"),
         d.dir("sub", [
-          d.file("main.dart", "main() => new StringBuffer().writeAll(['s']);")
+          d.file("main.dart", "main() => new Duration().toString();")
         ])
       ])
     ]).create();
@@ -33,17 +33,17 @@ main() {
       d.dir("build", [
         d.dir("web", [
           d.matcherFile("main.dart.js.map",
-              contains(r"packages/$sdk/lib/core/string_buffer.dart")),
+              contains(r"packages/$sdk/lib/core/duration.dart")),
           d.dir("sub", [
             d.matcherFile("main.dart.js.map",
-                contains(r"../packages/$sdk/lib/core/string_buffer.dart"))
+                contains(r"../packages/$sdk/lib/core/duration.dart"))
           ]),
           d.dir("packages", [
             d.dir(r"$sdk", [
               d.dir("lib", [
                 d.dir(r"core", [
-                  d.matcherFile("string_buffer.dart",
-                      contains("class StringBuffer"))
+                  d.matcherFile("duration.dart",
+                      contains("class Duration"))
                 ])
               ])
             ])