[wasm] Don't use SyncTextWriter in single-threaded wasm builds

src/libraries/System.Console/tests/ReadAndWrite.cs

@@ -158,7 +158,9 @@ public static async Task OutWriteAndWriteLineOverloads()
                 Console.SetOut(sw);
                 TextWriter writer = Console.Out;
                 Assert.NotNull(writer);
-                Assert.NotEqual(writer, sw); // the writer we provide gets wrapped
+                // Browser bypasses SyncTextWriter for faster startup
+                if (!OperatingSystem.IsBrowser())
+                    Assert.NotEqual(writer, sw); // the writer we provide gets wrapped
 
                 // We just want to ensure none of these throw exceptions, we don't actually validate
                 // what was written.

src/libraries/System.Console/tests/SyncTextWriter.cs

@@ -12,7 +12,8 @@
 
 public class SyncTextWriter
 {
-    [Fact]
+    // Browser bypasses SyncTextWriter for faster startup
+    [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
     public void SyncTextWriterLockedOnThis()
     {
         TextWriter oldWriter = Console.Out;

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -22,6 +22,8 @@
     <IsBigEndian Condition="'$(Platform)' == 's390x'">true</IsBigEndian>
     <Is64Bit Condition="'$(Platform)' == 'arm64' or '$(Platform)' == 'x64' or '$(Platform)' == 's390x' or '$(Platform)' == 'loongarch64' or '$(Platform)' == 'ppc64le' or '$(Platform)' == 'riscv64'">true</Is64Bit>
     <UseMinimalGlobalizationData Condition="'$(TargetsBrowser)' == 'true' or '$(TargetsWasi)' == 'true'">true</UseMinimalGlobalizationData>
+    <FeatureWasmManagedThreads Condition="'$(WasmEnableThreads)' == 'true'">true</FeatureWasmManagedThreads>
+    <DefineConstants Condition="'$(FeatureWasmManagedThreads)' == 'true'">$(DefineConstants);FEATURE_WASM_MANAGED_THREADS</DefineConstants>
   </PropertyGroup>
   <PropertyGroup>
     <DefineConstants Condition="'$(IsBigEndian)' == 'true'">$(DefineConstants);BIGENDIAN</DefineConstants>

src/libraries/System.Private.CoreLib/src/System/IO/TextWriter.cs

@@ -759,7 +759,11 @@ public static TextWriter Synchronized(TextWriter writer)
         {
             ArgumentNullException.ThrowIfNull(writer);
 
+#if !TARGET_BROWSER || FEATURE_WASM_MANAGED_THREADS
             return writer is SyncTextWriter ? writer : new SyncTextWriter(writer);
+#else
+            return writer;
+#endif
         }
 
         internal sealed class SyncTextWriter : TextWriter, IDisposable

src/libraries/System.Runtime/tests/System.IO.Tests/StreamReader/StreamReader.cs

@@ -29,7 +29,8 @@ public void ObjectClosedReadLineBaseStream()
             Assert.Throws<ObjectDisposedException>(() => sr.ReadLine());
         }
 
-        [Fact]
+        // Browser bypasses SyncTextWriter for faster startup
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
         public void Synchronized_NewObject()
         {
             using (Stream str = GetLargeStream())

src/libraries/System.Runtime/tests/System.IO.Tests/StreamWriter/StreamWriter.cs

@@ -7,7 +7,8 @@ namespace System.IO.Tests
 {
     public partial class WriteTests
     {
-        [Fact]
+        // Browser bypasses SyncTextWriter for faster startup
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
         public void Synchronized_NewObject()
         {
             using (Stream str = CreateStream())

src/libraries/System.Runtime/tests/System.IO.Tests/TextWriter/TextWriterTests.cs

@@ -690,7 +690,8 @@ public void DisposeAsync_ExceptionReturnedInTask()
             Assert.Same(e, vt.AsTask().Exception.InnerException);
         }
 
-        [Fact]
+        // Browser bypasses SyncTextWriter for faster startup
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
         public async Task FlushAsync_Precanceled()
         {
             Assert.Equal(TaskStatus.RanToCompletion, TextWriter.Null.FlushAsync(new CancellationToken(true)).Status);