Use the encoding parameter for writeString on RandomAccessFile objects.

tests/standalone/io/file_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 // Dart test program for testing file I/O.
 
 #import("dart:io");
 #import("dart:isolate");
 
 class MyListOfOneElement implements List {
@@ skipping 1200 matching lines @@
     openedFile.closeSync();
     openedFile = file.openSync(FileMode.WRITE);
     openedFile.setPositionSync(4);
     openedFile.writeStringSync("asdf");
     Expect.equals(8, openedFile.lengthSync());
     openedFile.closeSync();
     file.deleteSync();
     Expect.isFalse(file.existsSync());
   }
 
+  static void testWriteStringUtf8() {
+    var file = new File('${tempDirectory.path}/out_write_string');
+    var string = new String.fromCharCodes([0x192]);
+    file.open(FileMode.WRITE).then((openedFile) {
+      openedFile.writeString(string).then((_) {
+        openedFile.length().then((l) {
+          Expect.equals(2, l);
+          openedFile.close().then((_) {
+            file.open(FileMode.APPEND).then((openedFile) {
+              openedFile.setPosition(2).then((_) {
+                openedFile.writeString(string).then((_) {
+                  openedFile.length().then((l) {
+                    Expect.equals(4, l);
+                    openedFile.close().then((_) {
+                      file.readAsText().then((readBack) {
+                        Expect.stringEquals(readBack, '$string$string');
+                        file.delete().then((_) {
+                          file.exists().then((e) {
+                           Expect.isFalse(e);
+                           asyncTestDone("testWriteStringUtf8");
+                          });
+                        });
+                      });
+                    });
+                  });
+                });
+              });
+            });
+          });
+        });
+      });
+    });
+    asyncTestStarted();
+  }
+
+  static void testWriteStringUtf8Sync() {
+    var file = new File('${tempDirectory.path}/out_write_string_sync');
+    var string = new String.fromCharCodes([0x192]);
+    var openedFile = file.openSync(FileMode.WRITE);
+    openedFile.writeStringSync(string);
+    Expect.equals(2, openedFile.lengthSync());
+    openedFile.closeSync();
+    openedFile = file.openSync(FileMode.APPEND);
+    openedFile.setPositionSync(2);
+    openedFile.writeStringSync(string);
+    Expect.equals(4, openedFile.lengthSync());
+    openedFile.closeSync();
+    var readBack = file.readAsTextSync();
+    Expect.stringEquals(readBack, '$string$string');
+    file.deleteSync();
+    Expect.isFalse(file.existsSync());
+  }
+
   // Helper method to be able to run the test from the runtime
   // directory, or the top directory.
   static String getFilename(String path) =>
       new File(path).existsSync() ? path : 'runtime/$path';
 
   static String getDataFilename(String path) =>
       new File(path).existsSync() ? path : '../$path';
 
   // Main test entrypoint.
   static testMain() {
@@ skipping 35 matching lines @@
       testCloseExceptionStream();
       testBufferOutOfBoundsException();
       testAppend();
       testAppendSync();
       testWriteAppend();
       testOutputStreamWriteAppend();
       testOutputStreamWriteString();
       testWriteVariousLists();
       testDirectory();
       testDirectorySync();
+      testWriteStringUtf8();
+      testWriteStringUtf8Sync();
     });
   }
 }
 
 main() {
   FileTest.testMain();
 }