Add writeAsStringSync (issue 27109)

pkg/analyzer/test/file_system/memory_file_system_test.dart

Index: pkg/analyzer/test/file_system/memory_file_system_test.dart
diff --git a/pkg/analyzer/test/file_system/memory_file_system_test.dart b/pkg/analyzer/test/file_system/memory_file_system_test.dart
index 5c931b144489e8ba1ae3bcb187f4b4fd9d9d3159..2ee57753d04e487af650dc33206b92091ed1d43f 100644
--- a/pkg/analyzer/test/file_system/memory_file_system_test.dart
+++ b/pkg/analyzer/test/file_system/memory_file_system_test.dart
@@ -211,20 +211,43 @@ class FileTest {
   }
 
   void test_writeAsBytesSync_existing() {
-    File file = provider.newFileWithBytes('/foo/file.bin', <int>[1, 2]);
-    expect(file.readAsBytesSync(), <int>[1, 2]);
+    List<int> content = <int>[1, 2];
+    File file = provider.newFileWithBytes('/foo/file.bin', content);
+    expect(file.readAsBytesSync(), content);
     // write new bytes
-    file.writeAsBytesSync(<int>[10, 20]);
-    expect(file.readAsBytesSync(), <int>[10, 20]);
+    content = <int>[10, 20];
+    file.writeAsBytesSync(content);
+    expect(file.readAsBytesSync(), content);
   }
 
   void test_writeAsBytesSync_new() {
     File file = provider.getFile('/foo/file.bin');
     expect(file.exists, false);
     // write new bytes
-    file.writeAsBytesSync(<int>[10, 20]);
+    List<int> content = <int>[10, 20];
+    file.writeAsBytesSync(content);
     expect(file.exists, true);
-    expect(file.readAsBytesSync(), <int>[10, 20]);
+    expect(file.readAsBytesSync(), content);
+  }
+
+  void test_writeAsStringSync_existing() {
+    String content = 'ab';
+    File file = provider.newFile('/foo/file.txt', content);
+    expect(file.readAsStringSync(), content);
+    // write new bytes
+    content = 'CD';
+    file.writeAsStringSync(content);
+    expect(file.readAsStringSync(), content);
+  }
+
+  void test_writeAsStringSync_new() {
+    File file = provider.getFile('/foo/file.txt');
+    expect(file.exists, false);
+    // write new bytes
+    String content = 'ef';
+    file.writeAsStringSync(content);
+    expect(file.exists, true);
+    expect(file.readAsStringSync(), content);
   }
 }