Added MockFile.AppendText Implementation with unit test.

TestHelpers.Tests/MockFileTests.cs

@@ -1154,5 +1154,40 @@ public void MockFile_Open_OverwritesExistingFileOnCreate()
             Assert.That(stream.Length, Is.EqualTo(0));
             Assert.That(file.Contents.Length, Is.EqualTo(0));
         }
+        [Test]
+        public void MockFile_AppendText_AppendTextToanExistingFile()
+        {
+            const string filepath = @"c:\something\does\exist.txt";
+            var filesystem = new MockFileSystem(new Dictionary<string, MockFileData>
+            {
+                { filepath, new MockFileData("I'm here. ") }
+            });
+
+            var stream = filesystem.File.AppendText(filepath);
+
+            stream.Write("Me too!");
+            stream.Flush();
+            stream.Close();
+
+            var file = filesystem.GetFile(filepath);
+            Assert.That(file.TextContents, Is.EqualTo("I'm here. Me too!"));
+        }
+
+        [Test]
+        public void MockFile_AppendText_CreatesNewFileForAppendToNonExistingFile()
+        {
+            const string filepath = @"c:\something\doesnt\exist.txt";
+            var filesystem = new MockFileSystem(new Dictionary<string, MockFileData>());
+
+            var stream = filesystem.File.AppendText(filepath);
+
+            stream.Write("New too!");
+            stream.Flush();
+            stream.Close();
+
+            var file = filesystem.GetFile(filepath);
+            Assert.That(file.TextContents, Is.EqualTo("New too!"));
+            Assert.That(filesystem.FileExists(filepath));
+        }
     }
 }

TestHelpers.Tests/TestHelpers.Tests.csproj

@@ -117,6 +117,9 @@
     <None Include="packages.config" />
     <Compile Include="StringExtensionsTests.cs" />
   </ItemGroup>
+  <ItemGroup>
+    <Service Include="{82A7F48D-3B50-4B1E-B82E-3ADA8210C358}" />
+  </ItemGroup>
   <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
   <Import Project="$(SolutionDir)\.nuget\nuget.targets" />
   <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 

TestingHelpers/MockFile.cs

@@ -33,7 +33,17 @@ public override void AppendAllText(string path, string contents, Encoding encodi
 
         public override StreamWriter AppendText(string path)
         {
-            throw new NotImplementedException("This test helper hasn't been implemented yet. They are implemented on an as-needed basis. As it seems like you need it, now would be a great time to send us a pull request over at https://github.com/tathamoddie/System.IO.Abstractions. You know, because it's open source and all.");
+            if (mockFileDataAccessor.FileExists(path))
+            {
+                StreamWriter sw = new StreamWriter(OpenWrite(path));
+                sw.BaseStream.Seek(0, SeekOrigin.End); //push the stream pointer at the end for append.
+                return sw;
+
+            }
+            else
+            {
+                return new StreamWriter(Create(path));
+            }
         }
 
         public override void Copy(string sourceFileName, string destFileName)