Fix

Author: tmat

src/Compilers/CSharp/Portable/Symbols/Extensions/SourceExtensionImplementationMethodSymbol.cs

@@ -5,6 +5,7 @@
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Globalization;
+using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Threading;
 using Microsoft.CodeAnalysis.CSharp.Emit;