Allow formatting to not block on a background thread.

src/Workspaces/CSharp/Portable/Formatting/Engine/CSharpStructuredTriviaFormatEngine.cs

@@ -23,7 +23,7 @@ internal class CSharpStructuredTriviaFormatEngine : AbstractFormatEngine
         {
             var root = trivia.GetStructure();
             var formatter = new CSharpStructuredTriviaFormatEngine(trivia, initialColumn, optionSet, formattingRules, root.GetFirstToken(includeZeroWidth: true), root.GetLastToken(includeZeroWidth: true));
-            return formatter.FormatAsync(cancellationToken).WaitAndGetResult(cancellationToken);
+            return formatter.FormatAsync(cancellationToken).WaitAndGetResult_CanCallOnBackground(cancellationToken);
         }
 
         private CSharpStructuredTriviaFormatEngine(

src/Workspaces/CSharpTest/Formatting/FormattingElasticTriviaTests.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System.Linq;
+using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.Editing;
 using Microsoft.CodeAnalysis.Formatting;
@@ -14,7 +15,7 @@ public class FormattingEngineElasticTriviaTests : CSharpFormattingTestBase
     {
         [Fact(Skip = "530167")]
         [Trait(Traits.Feature, Traits.Features.Formatting)]
-        public void FormatElasticTrivia()
+        public async Task FormatElasticTrivia()
         {
             var expected = @"extern alias A1;
 
@@ -84,14 +85,14 @@ class B
 
             Assert.NotNull(compilation);
 
-            var newCompilation = Formatter.Format(compilation, new AdhocWorkspace());
+            var newCompilation = await Formatter.FormatAsync(compilation, new AdhocWorkspace());
             Assert.Equal(expected, newCompilation.ToFullString());
         }
 
         [WorkItem(1947, "https://github.com/dotnet/roslyn/issues/1947")]
         [Fact]
         [Trait(Traits.Feature, Traits.Features.Formatting)]
-        public void ElasticLineBreaksBetweenMembers()
+        public async Task ElasticLineBreaksBetweenMembers()
         {
             var text = @"
 public class C
@@ -125,20 +126,20 @@ public class C
 public class SomeAttribute : System.Attribute { }
 ";
 
-            var formatted = Formatter.Format(newRoot, ws).ToFullString();
+            var formatted = (await Formatter.FormatAsync(newRoot, ws)).ToFullString();
             Assert.Equal(expected, formatted);
 
-            var elasticOnlyFormatted = Formatter.Format(newRoot, SyntaxAnnotation.ElasticAnnotation, ws).ToFullString();
+            var elasticOnlyFormatted = (await Formatter.FormatAsync(newRoot, SyntaxAnnotation.ElasticAnnotation, ws)).ToFullString();
             Assert.Equal(expected, elasticOnlyFormatted);
 
-            var annotationFormatted = Formatter.Format(newRoot, Formatter.Annotation, ws).ToFullString();
+            var annotationFormatted = (await Formatter.FormatAsync(newRoot, Formatter.Annotation, ws)).ToFullString();
             Assert.Equal(expected, annotationFormatted);
         }
 
         [WorkItem(408, "https://roslyn.codeplex.com/workitem/408")]
         [Fact]
         [Trait(Traits.Feature, Traits.Features.Formatting)]
-        public void FormatElasticTriviaBetweenPropertiesWithoutAccessors()
+        public async Task FormatElasticTriviaBetweenPropertiesWithoutAccessors()
         {
             var expected = @"class PropertyTest
 {
@@ -187,7 +188,7 @@ public void FormatElasticTriviaBetweenPropertiesWithoutAccessors()
 
             Assert.NotNull(compilation);
 
-            var newCompilation = Formatter.Format(compilation, new AdhocWorkspace());
+            var newCompilation = await Formatter.FormatAsync(compilation, new AdhocWorkspace());
             Assert.Equal(expected, newCompilation.ToFullString());
         }
     }

src/Workspaces/CSharpTest/Formatting/FormattingMultipleSpanTests.cs

@@ -157,7 +157,7 @@ void F()
 
         [WorkItem(539231, "DevDiv")]
         [Fact, Trait(Traits.Feature, Traits.Features.Formatting)]
-        public void EmptySpan()
+        public async Task EmptySpan()
         {
             using (var workspace = new AdhocWorkspace())
             {
@@ -165,7 +165,7 @@ public void EmptySpan()
                 var document = project.AddDocument("Document", SourceText.From(""));
 
                 var syntaxTree = document.GetSyntaxTreeAsync().Result;
-                var result = Formatter.Format(syntaxTree.GetRoot(CancellationToken.None), TextSpan.FromBounds(0, 0), workspace, cancellationToken: CancellationToken.None);
+                var result = await Formatter.FormatAsync(syntaxTree.GetRoot(CancellationToken.None), TextSpan.FromBounds(0, 0), workspace, cancellationToken: CancellationToken.None);
             }
         }
 

src/Workspaces/CSharpTest/Formatting/FormattingTriviaTests.cs

@@ -1762,4 +1762,4 @@ class F
             Assert.Equal(expected, actual);
         }
     }
-}
+}
\ No newline at end of file