Annotate BoundNodeExtensions.

src/Compilers/CSharp/Portable/BoundTree/BoundNodeExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+#nullable enable
 
 using System.Collections.Immutable;
 using System.Diagnostics;
@@ -27,7 +28,7 @@ public static bool HasErrors<T>(this ImmutableArray<T> nodeArray)
         }
 
         // Like HasErrors property, but also returns false for a null node. 
-        public static bool HasErrors(this BoundNode node)
+        public static bool HasErrors(this BoundNode? node)
         {
             return node != null && node.HasErrors;
         }
@@ -35,7 +36,7 @@ public static bool HasErrors(this BoundNode node)
         public static bool IsConstructorInitializer(this BoundStatement statement)
         {
             Debug.Assert(statement != null);
-            if (statement.Kind == BoundKind.ExpressionStatement)
+            if (statement!.Kind == BoundKind.ExpressionStatement)
             {
                 BoundExpression expression = ((BoundExpressionStatement)statement).Expression;
                 if (expression.Kind == BoundKind.Sequence && ((BoundSequence)expression).SideEffects.IsDefaultOrEmpty)
@@ -53,8 +54,8 @@ public static bool IsConstructorInitializer(this BoundStatement statement)
         public static bool IsConstructorInitializer(this BoundCall call)
         {
             Debug.Assert(call != null);
-            MethodSymbol method = call.Method;
-            BoundExpression receiverOpt = call.ReceiverOpt;
+            MethodSymbol method = call!.Method;
+            BoundExpression? receiverOpt = call!.ReceiverOpt;
             return method.MethodKind == MethodKind.Constructor &&
                 receiverOpt != null &&
                 (receiverOpt.Kind == BoundKind.ThisReference || receiverOpt.Kind == BoundKind.BaseReference);