Update ref assemblies with readonly changes

Commit migrated from dotnet/corefx@45b724f

src/libraries/System.Collections.Specialized/ref/System.Collections.Specialized.cs

@@ -25,7 +25,7 @@ public partial struct BitVector32
         public override string ToString() { throw null; }
         public static string ToString(System.Collections.Specialized.BitVector32 value) { throw null; }
         [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
-        public partial struct Section
+        public readonly partial struct Section
         {
             public short Mask { get { throw null; } }
             public short Offset { get { throw null; } }

src/libraries/System.ComponentModel.TypeConverter/ref/System.ComponentModel.cs

@@ -1438,7 +1438,7 @@ public InstanceDescriptor(System.Reflection.MemberInfo member, System.Collection
         public object Invoke() { throw null; }
     }
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
-    public partial struct MemberRelationship
+    public readonly partial struct MemberRelationship
     {
         public static readonly System.ComponentModel.Design.Serialization.MemberRelationship Empty;
         public MemberRelationship(object owner, System.ComponentModel.MemberDescriptor member) { throw null; }

src/libraries/System.Console/ref/System.Console.cs

@@ -127,7 +127,7 @@ public enum ConsoleColor
         White = 15,
         Yellow = 14,
     }
-    public partial struct ConsoleKeyInfo
+    public readonly partial struct ConsoleKeyInfo
     {
         public ConsoleKeyInfo(char keyChar, ConsoleKey key, bool shift, bool alt, bool control) { }
         public char KeyChar { get { throw null; } }

src/libraries/System.Diagnostics.PerformanceCounter/ref/System.Diagnostics.PerformanceCounter.cs

@@ -32,7 +32,7 @@ protected override void OnValidate(object value) { }
         public virtual void Remove(System.Diagnostics.CounterCreationData value) { }
     }
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
-    public partial struct CounterSample
+    public readonly partial struct CounterSample
     {
         public static System.Diagnostics.CounterSample Empty;
         public CounterSample(long rawValue, long baseValue, long counterFrequency, long systemFrequency, long timeStamp, long timeStamp100nSec, System.Diagnostics.PerformanceCounterType counterType) { throw null;}

src/libraries/System.Diagnostics.StackTrace/ref/System.Diagnostics.StackTrace.cs

@@ -173,7 +173,7 @@ public enum SymAddressKind
         NativeStackRegister = 8,
     }
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
-    public partial struct SymbolToken
+    public readonly partial struct SymbolToken
     {
         public SymbolToken(int val) { throw null; }
         public bool Equals(System.Diagnostics.SymbolStore.SymbolToken obj) { throw null; }

src/libraries/System.Drawing.Primitives/ref/System.Drawing.Primitives.cs

@@ -7,7 +7,7 @@
 
 namespace System.Drawing
 {
-    public partial struct Color : System.IEquatable<System.Drawing.Color>
+    public readonly partial struct Color : System.IEquatable<System.Drawing.Color>
     {
         public static readonly System.Drawing.Color Empty;
         public byte A { get { throw null; } }

src/libraries/System.Net.Security/ref/System.Net.Security.cs

@@ -122,7 +122,7 @@ public partial class SslClientAuthenticationOptions
         public RemoteCertificateValidationCallback RemoteCertificateValidationCallback { get { throw null; } set { } }
         public EncryptionPolicy EncryptionPolicy { get { throw null; } set { } }
     }
-    public partial struct SslApplicationProtocol : IEquatable<SslApplicationProtocol>
+    public readonly partial struct SslApplicationProtocol : IEquatable<SslApplicationProtocol>
     {
         public static readonly SslApplicationProtocol Http2;
         public static readonly SslApplicationProtocol Http11;

src/libraries/System.Runtime.InteropServices.RuntimeInformation/ref/System.Runtime.InteropServices.RuntimeInformation.cs

@@ -9,7 +9,7 @@
 namespace System.Runtime.InteropServices
 {
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
-    public partial struct OSPlatform : System.IEquatable<System.Runtime.InteropServices.OSPlatform>
+    public readonly partial struct OSPlatform : System.IEquatable<System.Runtime.InteropServices.OSPlatform>
     {
         public static System.Runtime.InteropServices.OSPlatform Linux { get { throw null; } }
         public static System.Runtime.InteropServices.OSPlatform OSX { get { throw null; } }

src/libraries/System.Runtime.Numerics/ref/System.Runtime.Numerics.cs

@@ -9,7 +9,7 @@
 namespace System.Numerics
 {
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
-    public partial struct BigInteger : System.IComparable, System.IComparable<System.Numerics.BigInteger>, System.IEquatable<System.Numerics.BigInteger>, System.IFormattable
+    public readonly partial struct BigInteger : System.IComparable, System.IComparable<System.Numerics.BigInteger>, System.IEquatable<System.Numerics.BigInteger>, System.IFormattable
     {
         [System.CLSCompliantAttribute(false)]
         public BigInteger(byte[] value) { throw null; }

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -639,7 +639,7 @@ public CLSCompliantAttribute(bool isCompliant) { }
     }
     public delegate int Comparison<in T>(T x, T y);
     public delegate TOutput Converter<in TInput, out TOutput>(TInput input);
-    public partial struct DateTime : System.IComparable, System.IComparable<System.DateTime>, System.IConvertible, System.IEquatable<System.DateTime>, System.IFormattable, System.Runtime.Serialization.ISerializable
+    public readonly partial struct DateTime : System.IComparable, System.IComparable<System.DateTime>, System.IConvertible, System.IEquatable<System.DateTime>, System.IFormattable, System.Runtime.Serialization.ISerializable
     {
         public static readonly System.DateTime MaxValue;
         public static readonly System.DateTime MinValue;
@@ -4054,7 +4054,7 @@ public static class KeyValuePair
         public static KeyValuePair<TKey, TValue> Create<TKey, TValue>(TKey key, TValue value) { throw null; }        
     }
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
-    public partial struct KeyValuePair<TKey, TValue>
+    public readonly partial struct KeyValuePair<TKey, TValue>
     {
         public KeyValuePair(TKey key, TValue value) { throw null; }
         [System.ComponentModel.EditorBrowsable(System.ComponentModel.EditorBrowsableState.Never)]
@@ -6165,7 +6165,7 @@ protected ParameterInfo() { }
         public override string ToString() { throw null; }
     }
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
-    public struct ParameterModifier
+    public readonly struct ParameterModifier
     {
         public ParameterModifier(int parameterCount) { }
         public bool this[int index] { get { throw null; } set { } }
@@ -6564,10 +6564,10 @@ public void Clear() { }
         public bool TryGetValue(TKey key, out TValue value) { throw null; }
         public delegate TValue CreateValueCallback(TKey key);
     }
-    public partial struct ConfiguredValueTaskAwaitable<TResult>
+    public readonly partial struct ConfiguredValueTaskAwaitable<TResult>
     {
         public System.Runtime.CompilerServices.ConfiguredValueTaskAwaitable<TResult>.ConfiguredValueTaskAwaiter GetAwaiter() { throw null; }
-        public partial struct ConfiguredValueTaskAwaiter : System.Runtime.CompilerServices.ICriticalNotifyCompletion, System.Runtime.CompilerServices.INotifyCompletion
+        public readonly partial struct ConfiguredValueTaskAwaiter : System.Runtime.CompilerServices.ICriticalNotifyCompletion, System.Runtime.CompilerServices.INotifyCompletion
         {
             public bool IsCompleted { get { throw null; } }
             public TResult GetResult() { throw null; }
@@ -6751,7 +6751,7 @@ public sealed partial class UnsafeValueTypeAttribute : System.Attribute
         public UnsafeValueTypeAttribute() { }
     }
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
-    public partial struct ValueTaskAwaiter<TResult> : System.Runtime.CompilerServices.ICriticalNotifyCompletion, System.Runtime.CompilerServices.INotifyCompletion
+    public readonly partial struct ValueTaskAwaiter<TResult> : System.Runtime.CompilerServices.ICriticalNotifyCompletion, System.Runtime.CompilerServices.INotifyCompletion
     {
         public bool IsCompleted { get { throw null; } }
         public TResult GetResult() { throw null; }
@@ -7060,7 +7060,7 @@ private SerializationInfoEnumerator() { }
         public void Reset() { throw null; }
     }
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
-    public partial struct StreamingContext
+    public readonly partial struct StreamingContext
     {
         public StreamingContext(System.Runtime.Serialization.StreamingContextStates state) { }
         public StreamingContext(System.Runtime.Serialization.StreamingContextStates state, object additional) { }
@@ -7997,7 +7997,7 @@ public UnobservedTaskExceptionEventArgs(System.AggregateException exception) { }
         public void SetObserved() { }
     }
     [System.Runtime.CompilerServices.AsyncMethodBuilderAttribute(typeof(System.Runtime.CompilerServices.AsyncValueTaskMethodBuilder<>))]
-    public partial struct ValueTask<TResult> : System.IEquatable<System.Threading.Tasks.ValueTask<TResult>>
+    public readonly partial struct ValueTask<TResult> : System.IEquatable<System.Threading.Tasks.ValueTask<TResult>>
     {
         public ValueTask(System.Threading.Tasks.Task<TResult> task) { throw null; }
         public ValueTask(TResult result) { throw null; }

src/libraries/System.Security.Cryptography.Primitives/ref/System.Security.Cryptography.Primitives.cs

@@ -103,7 +103,7 @@ protected virtual void Dispose(bool disposing) { }
         protected virtual bool TryHashFinal(Span<byte> destination, out int bytesWritten) { throw null; }
     }
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
-    public partial struct HashAlgorithmName : System.IEquatable<System.Security.Cryptography.HashAlgorithmName>
+    public readonly partial struct HashAlgorithmName : System.IEquatable<System.Security.Cryptography.HashAlgorithmName>
     {
         public HashAlgorithmName(string name) { throw null; }
         public static System.Security.Cryptography.HashAlgorithmName MD5 { get { throw null; } }

src/libraries/System.ServiceProcess.ServiceController/ref/System.ServiceProcess.ServiceController.cs

@@ -116,7 +116,7 @@ public enum ServiceType
         Win32ShareProcess = 32,
     }
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
-    public partial struct SessionChangeDescription
+    public readonly partial struct SessionChangeDescription
     {
         public System.ServiceProcess.SessionChangeReason Reason { get { throw null; } }
         public int SessionId { get { throw null; } }

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

@@ -159,7 +159,7 @@ public DataflowLinkOptions() { }
         public bool PropagateCompletion { get { throw null; } set { } }
     }
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
-    public partial struct DataflowMessageHeader : System.IEquatable<System.Threading.Tasks.Dataflow.DataflowMessageHeader>
+    public readonly partial struct DataflowMessageHeader : System.IEquatable<System.Threading.Tasks.Dataflow.DataflowMessageHeader>
     {
         public DataflowMessageHeader(long id) { throw null;}
         public long Id { get { throw null; } }