Moved file checks to a filter

Moved the checks for the existance of the file and having a non-readonly
nature to an actual IFormattingFilter instance.

Author: jaredpar

src/Microsoft.DotNet.CodeFormatting/Filters/UsableFileFilter.cs

@@ -0,0 +1,42 @@
+using Microsoft.CodeAnalysis;
+using System;
+using System.Collections.Generic;
+using System.ComponentModel.Composition;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Microsoft.DotNet.CodeFormatting.Filters
+{
+    [Export(typeof(IFormattingFilter))]
+    internal sealed class UsableFileFilter : IFormattingFilter
+    {
+        private readonly Options _options;
+
+        [ImportingConstructor]
+        internal UsableFileFilter(Options options)
+        {
+            _options = options;
+        }
+
+        public bool ShouldBeProcessed(Document document)
+        {
+            if (document.FilePath == null)
+            {
+                return true;
+            }
+
+            var fileInfo = new FileInfo(document.FilePath);
+            if (!fileInfo.Exists || fileInfo.IsReadOnly)
+            {
+                _options.FormatLogger.WriteLine("warning: skipping document '{0}' because it {1}.",
+                    document.FilePath,
+                    fileInfo.IsReadOnly ? "is read-only" : "does not exist");
+                return false;
+            }
+
+            return true;
+        }
+    }
+}

src/Microsoft.DotNet.CodeFormatting/FormattingEngineImplementation.cs

@@ -127,18 +127,6 @@ private async Task SaveChanges(Solution solution, Solution originalSolution, Can
 
         private bool ShouldBeProcessed(Document document)
         {
-            if (document.FilePath != null)
-            {
-                var fileInfo = new FileInfo(document.FilePath);
-                if (!fileInfo.Exists || fileInfo.IsReadOnly)
-                {
-                    FormatLogger.WriteLine("warning: skipping document '{0}' because it {1}.",
-                        document.FilePath,
-                        fileInfo.IsReadOnly ? "is read-only" : "does not exist");
-                    return false;
-                }
-            }
-
             foreach (var filter in _filters)
             {
                 var shouldBeProcessed = filter.ShouldBeProcessed(document);

src/Microsoft.DotNet.CodeFormatting/Microsoft.DotNet.CodeFormatting.csproj

@@ -72,6 +72,7 @@
   </ItemGroup>
   <ItemGroup>
     <Compile Include="Filters\FilenameFilter.cs" />
+    <Compile Include="Filters\UsableFileFilter.cs" />
     <Compile Include="FormattingEngine.cs" />
     <Compile Include="FormattingEngineImplementation.cs" />
     <Compile Include="IFormatLogger.cs" />