Revert old behaviour of `ReadToEnd`

osu.Game.Tests/Beatmaps/IO/LineBufferedReaderTest.cs

@@ -108,13 +108,19 @@ public void TestReadToEndNoPeeks()
         [Test]
         public void TestReadToEndAfterReadsAndPeeks()
         {
-            const string contents = "first line\r\nsecond line";
+            const string contents = "this line is gone\rthis one shouldn't be\r\nthese ones\ndefinitely not";
 
             using (var stream = new MemoryStream(Encoding.UTF8.GetBytes(contents)))
             using (var bufferedReader = new LineBufferedReader(stream))
             {
-                bufferedReader.PeekLine();
-                Assert.Throws<InvalidOperationException>(() => bufferedReader.ReadToEnd());
+                Assert.AreEqual("this line is gone", bufferedReader.ReadLine());
+                Assert.AreEqual("this one shouldn't be", bufferedReader.PeekLine());
+
+                string[] endingLines = bufferedReader.ReadToEnd().Split(new[] { '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries);
+                Assert.AreEqual(3, endingLines.Length);
+                Assert.AreEqual("this one shouldn't be", endingLines[0]);
+                Assert.AreEqual("these ones", endingLines[1]);
+                Assert.AreEqual("definitely not", endingLines[2]);
             }
         }
     }

osu.Game/IO/LineBufferedReader.cs

@@ -46,14 +46,21 @@ public LineBufferedReader(Stream stream, bool leaveOpen = false)
 
         /// <summary>
         /// Reads the stream to its end and returns the text read.
-        /// Not compatible with calls to <see cref="PeekLine"/>.
+        /// This includes any peeked but unconsumed lines.
         /// </summary>
         public string ReadToEnd()
         {
-            if (peekedLine != null)
-                throw new InvalidOperationException($"Do not use {nameof(ReadToEnd)} when also peeking for lines.");
+            string remainingText = streamReader.ReadToEnd();
+            if (peekedLine == null)
+                return remainingText;
 
-            return streamReader.ReadToEnd();
+            var builder = new StringBuilder();
+
+            // this might not be completely correct due to varying platform line endings
+            builder.AppendLine(peekedLine);
+            builder.Append(remainingText);
+
+            return builder.ToString();
         }
 
         public void Dispose()