Make SourceText's CalculateChecksum method private

src/Compilers/Core/Portable/PublicAPI.txt

@@ -2024,7 +2024,6 @@ static Microsoft.CodeAnalysis.Text.LinePosition.operator >(Microsoft.CodeAnalysi
 static Microsoft.CodeAnalysis.Text.LinePosition.operator >=(Microsoft.CodeAnalysis.Text.LinePosition left, Microsoft.CodeAnalysis.Text.LinePosition right) -> bool
 static Microsoft.CodeAnalysis.Text.LinePositionSpan.operator !=(Microsoft.CodeAnalysis.Text.LinePositionSpan left, Microsoft.CodeAnalysis.Text.LinePositionSpan right) -> bool
 static Microsoft.CodeAnalysis.Text.LinePositionSpan.operator ==(Microsoft.CodeAnalysis.Text.LinePositionSpan left, Microsoft.CodeAnalysis.Text.LinePositionSpan right) -> bool
-static Microsoft.CodeAnalysis.Text.SourceText.CalculateChecksum(byte[] buffer, int offset, int count, Microsoft.CodeAnalysis.Text.SourceHashAlgorithm algorithmId) -> System.Collections.Immutable.ImmutableArray<byte>
 static Microsoft.CodeAnalysis.Text.SourceText.From(System.IO.Stream stream, System.Text.Encoding encoding = null, Microsoft.CodeAnalysis.Text.SourceHashAlgorithm checksumAlgorithm = Microsoft.CodeAnalysis.Text.SourceHashAlgorithm.Sha1, bool throwIfBinaryDetected = false) -> Microsoft.CodeAnalysis.Text.SourceText
 static Microsoft.CodeAnalysis.Text.SourceText.From(byte[] buffer, int length, System.Text.Encoding encoding = null, Microsoft.CodeAnalysis.Text.SourceHashAlgorithm checksumAlgorithm = Microsoft.CodeAnalysis.Text.SourceHashAlgorithm.Sha1, bool throwIfBinaryDetected = false) -> Microsoft.CodeAnalysis.Text.SourceText
 static Microsoft.CodeAnalysis.Text.SourceText.From(string text, System.Text.Encoding encoding = null, Microsoft.CodeAnalysis.Text.SourceHashAlgorithm checksumAlgorithm = Microsoft.CodeAnalysis.Text.SourceHashAlgorithm.Sha1) -> Microsoft.CodeAnalysis.Text.SourceText

src/Compilers/Core/Portable/Text/SourceText.cs

@@ -425,7 +425,7 @@ private static ImmutableArray<byte> CalculateChecksum(Stream stream, SourceHashA
             }
         }
 
-        protected static ImmutableArray<byte> CalculateChecksum(byte[] buffer, int offset, int count, SourceHashAlgorithm algorithmId)
+        private static ImmutableArray<byte> CalculateChecksum(byte[] buffer, int offset, int count, SourceHashAlgorithm algorithmId)
         {
             using (var algorithm = CryptographicHashProvider.TryGetAlgorithm(algorithmId))
             {