fixed conversions tests failing by adding a polyfill for the frameworks that do not have the nullability attributes

ICSharpCode.Decompiler/Util/CSharpPrimitiveCast.cs

@@ -35,8 +35,10 @@ namespace ICSharpCode.Decompiler.Util
 		/// </summary>
 		/// <exception cref="OverflowException">Overflow checking is enabled and an overflow occurred.</exception>
 		/// <exception cref="InvalidCastException">The cast is invalid, e.g. casting a boolean to an integer.</exception>
-		//[return: MaybeNull]
-		//[return: NotNullIfNotNull(nameof(input))]
+		/// 
+
+		[return: MaybeNull]
+		[return: NotNullIfNotNull("input")]
 		public static object Cast(TypeCode targetType, [MaybeNull] object input, bool checkForOverflow)
 		{
 			if (input == null)
@@ -772,3 +774,35 @@ namespace ICSharpCode.Decompiler.Util
 		}
 	}
 }
+#if (!NETSTANDARD2_0_OR_GREATER && !NETCORE) || MCS
+namespace System.Diagnostics.CodeAnalysis
+{
+	/// <summary>Specifies that the output will be non-null if the named parameter is non-null.</summary>
+	[AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, AllowMultiple = true, Inherited = false)]
+	internal sealed class NotNullIfNotNullAttribute : Attribute
+	{
+		/// <summary>Initializes the attribute with the associated parameter name.</summary>
+		/// <param name="parameterName">
+		/// The associated parameter name.  The output will be non-null if the argument to the parameter specified is non-null.
+		/// </param>
+		public NotNullIfNotNullAttribute(string parameterName)
+		{
+			ParameterName = parameterName;
+		}
+		private string _parameterName;
+		/// <summary>Gets the associated parameter name.</summary>
+		public string ParameterName {
+			get {
+				return _parameterName;
+			}
+			private set {
+				_parameterName = value;
+			}
+		}
+	}
+
+	/// <summary>Specifies that an output may be null even if the corresponding type disallows it.</summary>
+	[AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+	internal sealed class MaybeNullAttribute : Attribute { }
+}
+#endif