Change FileTextLoader to open FileStream for asynchronous IO. (changeset 1355769)

Src/Compilers/Core/Desktop/FileUtilities.cs

@@ -296,6 +296,24 @@ internal static FileStream OpenRead(string fullPath)
             }
         }
 
+        internal static FileStream OpenAsyncRead(string fullPath)
+        {
+            Debug.Assert(PathUtilities.IsAbsolute(fullPath));
+
+            try
+            {
+                return new FileStream(fullPath, FileMode.Open, FileAccess.Read, FileShare.Read, 4096, FileOptions.Asynchronous);
+            }
+            catch (IOException)
+            {
+                throw;
+            }
+            catch (Exception e)
+            {
+                throw new IOException(e.Message, e);
+            }
+        }
+
         /// <summary>
         /// Used to create a file given a path specified by the user.
         /// paramName - Provided by the Public surface APIs to have a clearer message. Internal API just rethrow the exception

Src/Workspaces/Core/Desktop/Workspace/FileTextLoader.cs

@@ -70,8 +70,9 @@ public override async Task<TextAndVersion> LoadTextAndVersionAsync(Workspace wor
             // Allowing other theads/processes to write or delete the file is essential for scenarios such as
             // Rename refactoring where File.Replace API is invoked for updating the modified file. 
             TextAndVersion textAndVersion;
-            using (var stream = FileUtilities.OpenRead(this.path))
+            using (var stream = FileUtilities.OpenAsyncRead(this.path))
             {
+                System.Diagnostics.Debug.Assert(stream.IsAsync);
                 var version = VersionStamp.Create(prevLastWriteTime);
                 var memoryStream = await this.ReadStreamAsync(stream, cancellationToken: cancellationToken).ConfigureAwait(false);
 

Src/Workspaces/Core/Desktop/Workspace/MSBuild/ProjectFile/ProjectFileLoader.cs

@@ -70,7 +70,7 @@ private static async Task<MemoryStream> ReadFileAsync(string path, CancellationT
         {
             MemoryStream memoryStream = new MemoryStream();
             var buffer = new byte[1024];
-            using (var stream = FileUtilities.OpenRead(path))
+            using (var stream = FileUtilities.OpenAsyncRead(path))
             {
                 int bytesRead = 0;
                 do