Remove synchronous work in Change-Signature.

src/Features/CSharp/Portable/ChangeSignature/CSharpChangeSignatureService.cs

@@ -23,9 +23,10 @@ namespace Microsoft.CodeAnalysis.CSharp.ChangeSignature
     [ExportLanguageService(typeof(AbstractChangeSignatureService), LanguageNames.CSharp), Shared]
     internal sealed class CSharpChangeSignatureService : AbstractChangeSignatureService
     {
-        public override ISymbol GetInvocationSymbol(Document document, int position, bool restrictToDeclarations, CancellationToken cancellationToken)
+        public override async Task<ISymbol> GetInvocationSymbolAsync(
+            Document document, int position, bool restrictToDeclarations, CancellationToken cancellationToken)
         {
-            var tree = document.GetSyntaxTreeAsync(cancellationToken).WaitAndGetResult(cancellationToken);
+            var tree = await document.GetSyntaxTreeAsync(cancellationToken).ConfigureAwait(false);
             var token = tree.GetRoot(cancellationToken).FindToken(position != tree.Length ? position : Math.Max(0, position - 1));
 
             var ancestorDeclarationKinds = restrictToDeclarations ? _invokableAncestorKinds.Add(SyntaxKind.Block) : _invokableAncestorKinds;

src/Features/Core/Portable/ChangeSignature/AbstractChangeSignatureService.cs

@@ -27,7 +27,7 @@ internal abstract class AbstractChangeSignatureService : ILanguageService
         /// <summary>
         /// Determines the symbol on which we are invoking ReorderParameters
         /// </summary>
-        public abstract ISymbol GetInvocationSymbol(Document document, int position, bool restrictToDeclarations, CancellationToken cancellationToken);
+        public abstract Task<ISymbol> GetInvocationSymbolAsync(Document document, int position, bool restrictToDeclarations, CancellationToken cancellationToken);
 
         /// <summary>
         /// Given a SyntaxNode for which we want to reorder parameters/arguments, find the 
@@ -85,7 +85,7 @@ internal ChangeSignatureResult ChangeSignature(Document document, int position,
 
         private async Task<ChangeSignatureAnalyzedContext> GetContextAsync(Document document, int position, bool restrictToDeclarations, CancellationToken cancellationToken)
         {
-            var symbol = GetInvocationSymbol(document, position, restrictToDeclarations, cancellationToken);
+            var symbol = await GetInvocationSymbolAsync(document, position, restrictToDeclarations, cancellationToken).ConfigureAwait(false);
 
             // Cross-lang symbols will show as metadata, so map it to source if possible.
             symbol = await SymbolFinder.FindSourceDefinitionAsync(symbol, document.Project.Solution, cancellationToken).ConfigureAwait(false) ?? symbol;

src/Features/VisualBasic/Portable/ChangeSignature/VisualBasicChangeSignatureService.vb

@@ -17,8 +17,12 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.ChangeSignature
     Friend Class VisualBasicChangeSignatureService
         Inherits AbstractChangeSignatureService
 
-        Public Overrides Function GetInvocationSymbol(document As Document, position As Integer, restrictToDeclarations As Boolean, cancellationToken As CancellationToken) As ISymbol
-            Dim tree = document.GetSyntaxTreeAsync(cancellationToken).WaitAndGetResult(cancellationToken)
+        Public Overrides Async Function GetInvocationSymbolAsync(
+                document As Document,
+                position As Integer,
+                restrictToDeclarations As Boolean,
+                cancellationToken As CancellationToken) As Task(Of ISymbol)
+            Dim tree = Await document.GetSyntaxTreeAsync(cancellationToken).ConfigureAwait(False)
             Dim token = tree.GetRoot(cancellationToken).FindToken(If(position <> tree.Length, position, Math.Max(0, position - 1)))
 
             Dim matchingNode = token.Parent.AncestorsAndSelf().FirstOrDefault(Function(n) _invokableAncestorKinds.Contains(n.Kind))