fixed test failures.

allow SourceText From TextReader to have null encoding lke the  one from string.

src/Compilers/Core/CodeAnalysisTest/Text/SourceTextTests.cs

@@ -240,11 +240,9 @@ public void FromTextReader()
 
             Assert.Equal<byte>(expectedSourceText.GetChecksum(), actualSourceText.GetChecksum());
 
-            var utf8NoBOM = new UTF8Encoding(encoderShouldEmitUTF8Identifier: false);
-
             Assert.Same(s_utf8, SourceText.From(actual, expected.Length, s_utf8).Encoding);
             Assert.Same(s_unicode, SourceText.From(actual, expected.Length, s_unicode).Encoding);
-            Assert.Equal(utf8NoBOM, SourceText.From(actual, expected.Length, null).Encoding);
+            Assert.Null(SourceText.From(actual, expected.Length, null).Encoding);
         }
 
         [Fact]
@@ -263,7 +261,7 @@ public void FromTextReader_Large()
 
             Assert.Same(s_utf8, SourceText.From(actual, expected.Length, s_utf8).Encoding);
             Assert.Same(s_unicode, SourceText.From(actual, expected.Length, s_unicode).Encoding);
-            Assert.Equal(utf8NoBOM, SourceText.From(actual, expected.Length, null).Encoding);
+            Assert.Null(SourceText.From(actual, expected.Length, null).Encoding);
         }
 
         private static void TestTryReadByteOrderMark(Encoding expectedEncoding, int expectedPreambleLength, byte[] data)

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

@@ -24,13 +24,13 @@ internal sealed class LargeText : SourceText
         private readonly ImmutableArray<char[]> _chunks;
         private readonly int[] _chunkStartOffsets;
         private readonly int _length;
-        private readonly Encoding _encoding;
+        private readonly Encoding _encodingOpt;
 
-        internal LargeText(ImmutableArray<char[]> chunks, Encoding encoding, ImmutableArray<byte> checksum, SourceHashAlgorithm checksumAlgorithm, ImmutableArray<byte> embeddedTextBlob)
+        internal LargeText(ImmutableArray<char[]> chunks, Encoding encodingOpt, ImmutableArray<byte> checksum, SourceHashAlgorithm checksumAlgorithm, ImmutableArray<byte> embeddedTextBlob)
             : base(checksum, checksumAlgorithm, embeddedTextBlob)
         {
             _chunks = chunks;
-            _encoding = encoding;
+            _encodingOpt = encodingOpt;
             _chunkStartOffsets = new int[chunks.Length];
 
             int offset = 0;
@@ -43,8 +43,8 @@ internal LargeText(ImmutableArray<char[]> chunks, Encoding encoding, ImmutableAr
             _length = offset;
         }
 
-        internal LargeText(ImmutableArray<char[]> chunks, Encoding encoding, SourceHashAlgorithm checksumAlgorithm)
-            : this(chunks, encoding, default(ImmutableArray<byte>), checksumAlgorithm, default(ImmutableArray<byte>))
+        internal LargeText(ImmutableArray<char[]> chunks, Encoding encodingOpt, SourceHashAlgorithm checksumAlgorithm)
+            : this(chunks, encodingOpt, default(ImmutableArray<byte>), checksumAlgorithm, default(ImmutableArray<byte>))
         {
         }
 
@@ -74,17 +74,17 @@ internal static SourceText Decode(Stream stream, Encoding encoding, SourceHashAl
             }
         }
 
-        internal static SourceText Decode(TextReader reader, int length, Encoding encoding, SourceHashAlgorithm checksumAlgorithm)
+        internal static SourceText Decode(TextReader reader, int length, Encoding encodingOpt, SourceHashAlgorithm checksumAlgorithm)
         {
             if (length == 0)
             {
-                return SourceText.From(string.Empty, encoding, checksumAlgorithm);
+                return SourceText.From(string.Empty, encodingOpt, checksumAlgorithm);
             }
 
             // throwIfBinaryDetected == false since we are given text reader from the beginning
             var chunks = ReadFromTextReader(reader, length, throwIfBinaryDetected: false);
 
-            return new LargeText(chunks, encoding, checksumAlgorithm);
+            return new LargeText(chunks, encodingOpt, checksumAlgorithm);
         }
 
         private static ImmutableArray<char[]> ReadFromTextReader(TextReader reader, int maxCharRemainingGuess, bool throwIfBinaryDetected)
@@ -173,7 +173,7 @@ private int GetIndexFromPosition(int position)
             }
         }
 
-        public override Encoding Encoding => _encoding;
+        public override Encoding Encoding => _encodingOpt;
 
         public override int Length => _length;
 

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

@@ -124,10 +124,6 @@ public static SourceText From(string text, Encoding encoding = null, SourceHashA
                 throw new ArgumentNullException(nameof(reader));
             }
 
-            ValidateChecksumAlgorithm(checksumAlgorithm);
-
-            encoding = encoding ?? s_utf8EncodingWithNoBOM;
-
             // If the resulting string would end up on the large object heap, then use LargeEncodedText.
             if (length >= LargeObjectHeapLimitInChars)
             {