Mark some APIs throwing PNSE from runtime as unsupported on Browser

src/coreclr/src/System.Private.CoreLib/src/System/Threading/Monitor.cs

@@ -12,10 +12,11 @@
 **
 =============================================================================*/
 
-using System.Runtime.CompilerServices;
 using System.Diagnostics;
-using System.Runtime.InteropServices;
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
 
 namespace System.Threading
 {
@@ -170,28 +171,33 @@ public static bool IsEntered(object obj)
         [MethodImpl(MethodImplOptions.InternalCall)]
         private static extern bool ObjWait(bool exitContext, int millisecondsTimeout, object obj);
 
+        [UnsupportedOSPlatform("browser")]
         public static bool Wait(object obj, int millisecondsTimeout, bool exitContext)
         {
             if (obj == null)
                 throw (new ArgumentNullException(nameof(obj)));
             return ObjWait(exitContext, millisecondsTimeout, obj);
         }
 
+        [UnsupportedOSPlatform("browser")]
         public static bool Wait(object obj, TimeSpan timeout, bool exitContext)
         {
             return Wait(obj, MillisecondsTimeoutFromTimeSpan(timeout), exitContext);
         }
 
+        [UnsupportedOSPlatform("browser")]
         public static bool Wait(object obj, int millisecondsTimeout)
         {
             return Wait(obj, millisecondsTimeout, false);
         }
 
+        [UnsupportedOSPlatform("browser")]
         public static bool Wait(object obj, TimeSpan timeout)
         {
             return Wait(obj, MillisecondsTimeoutFromTimeSpan(timeout), false);
         }
 
+        [UnsupportedOSPlatform("browser")]
         public static bool Wait(object obj)
         {
             return Wait(obj, Timeout.Infinite, false);

src/coreclr/src/System.Private.CoreLib/src/System/Threading/Thread.CoreCLR.cs

@@ -6,6 +6,7 @@
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Runtime.Serialization;
+using System.Runtime.Versioning;
 
 namespace System.Threading
 {
@@ -182,6 +183,7 @@ internal ThreadHandle GetNativeHandle()
         /// method on the IThreadable interface passed in the constructor. Once the
         /// thread is dead, it cannot be restarted with another call to Start.
         /// </summary>
+        [UnsupportedOSPlatform("browser")]
         public void Start(object? parameter)
         {
             // In the case of a null delegate (second call to start on same thread)
@@ -196,6 +198,7 @@ public void Start(object? parameter)
             Start();
         }
 
+        [UnsupportedOSPlatform("browser")]
         public void Start()
         {
 #if FEATURE_COMINTEROP_APARTMENT_SUPPORT

src/libraries/System.Threading.Thread/ref/System.Threading.Thread.cs

@@ -89,7 +89,9 @@ public static void SetData(System.LocalDataStoreSlot slot, object? data) { }
         public static void Sleep(int millisecondsTimeout) { }
         public static void Sleep(System.TimeSpan timeout) { }
         public static void SpinWait(int iterations) { }
+        [System.Runtime.Versioning.UnsupportedOSPlatformAttribute("browser")]
         public void Start() { }
+        [System.Runtime.Versioning.UnsupportedOSPlatformAttribute("browser")]
         public void Start(object? parameter) { }
         [System.ObsoleteAttribute("Thread.Suspend has been deprecated.  Please use other classes in System.Threading, such as Monitor, Mutex, Event, and Semaphore, to synchronize Threads or protect resources.  https://go.microsoft.com/fwlink/?linkid=14202", false)]
         public void Suspend() { }

src/libraries/System.Threading/ref/System.Threading.cs

@@ -280,10 +280,15 @@ public static void TryEnter(object obj, ref bool lockTaken) { }
         public static void TryEnter(object obj, int millisecondsTimeout, ref bool lockTaken) { }
         public static bool TryEnter(object obj, System.TimeSpan timeout) { throw null; }
         public static void TryEnter(object obj, System.TimeSpan timeout, ref bool lockTaken) { }
+        [System.Runtime.Versioning.UnsupportedOSPlatformAttribute("browser")]
         public static bool Wait(object obj) { throw null; }
+        [System.Runtime.Versioning.UnsupportedOSPlatformAttribute("browser")]
         public static bool Wait(object obj, int millisecondsTimeout) { throw null; }
+        [System.Runtime.Versioning.UnsupportedOSPlatformAttribute("browser")]
         public static bool Wait(object obj, int millisecondsTimeout, bool exitContext) { throw null; }
+        [System.Runtime.Versioning.UnsupportedOSPlatformAttribute("browser")]
         public static bool Wait(object obj, System.TimeSpan timeout) { throw null; }
+        [System.Runtime.Versioning.UnsupportedOSPlatformAttribute("browser")]
         public static bool Wait(object obj, System.TimeSpan timeout, bool exitContext) { throw null; }
     }
     public sealed partial class Mutex : System.Threading.WaitHandle

src/mono/netcore/System.Private.CoreLib/src/System/Threading/Monitor.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Runtime.CompilerServices;
+using System.Runtime.Versioning;
 
 namespace System.Threading
 {
@@ -80,19 +81,24 @@ public static bool IsEntered(object obj)
             return IsEnteredNative(obj);
         }
 
+        [UnsupportedOSPlatform("browser")]
         public static bool Wait(object obj, int millisecondsTimeout, bool exitContext)
         {
             if (obj == null)
                 throw new ArgumentNullException(nameof(obj));
             return ObjWait(exitContext, millisecondsTimeout, obj);
         }
 
+        [UnsupportedOSPlatform("browser")]
         public static bool Wait(object obj, TimeSpan timeout, bool exitContext) => Wait(obj, MillisecondsTimeoutFromTimeSpan(timeout), exitContext);
 
+        [UnsupportedOSPlatform("browser")]
         public static bool Wait(object obj, int millisecondsTimeout) => Wait(obj, millisecondsTimeout, false);
 
+        [UnsupportedOSPlatform("browser")]
         public static bool Wait(object obj, TimeSpan timeout) => Wait(obj, MillisecondsTimeoutFromTimeSpan(timeout), false);
 
+        [UnsupportedOSPlatform("browser")]
         public static bool Wait(object obj) => Wait(obj, Timeout.Infinite, false);
 
         public static void Pulse(object obj)

src/mono/netcore/System.Private.CoreLib/src/System/Threading/Thread.Mono.cs

@@ -5,6 +5,7 @@
 using System.Globalization;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
 
 namespace System.Threading
 {
@@ -251,11 +252,13 @@ public static void Sleep(int millisecondsTimeout)
 
         internal static void UninterruptibleSleep0() => SleepInternal(0, false);
 
+        [UnsupportedOSPlatform("browser")]
         public void Start()
         {
             StartInternal(this);
         }
 
+        [UnsupportedOSPlatform("browser")]
         public void Start(object parameter)
         {
             if (m_start is ThreadStart)