Add Conditional Target capabilities to the build scripts

Make Attribute properties immutable

PR Feedback, bugfixes

PR Feedback

scripts/Microsoft.DotNet.Cli.Build.Framework/TargetConditions/BuildPlatformsAttribute.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Collections.Generic;
+
+namespace Microsoft.DotNet.Cli.Build.Framework
+{
+    [AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
+    public class BuildPlatformsAttribute : TargetConditionAttribute
+    {
+        private IEnumerable<BuildPlatform> _buildPlatforms;
+
+        public BuildPlatformsAttribute(params BuildPlatform[] platforms)
+        {
+            if (platforms == null)
+            {
+                throw new ArgumentNullException("platforms");
+            }
+
+            _buildPlatforms = platforms;
+        }
+
+        public override bool EvaluateCondition()
+        {
+            var currentPlatform = CurrentPlatform.Current;
+
+            if (currentPlatform == default(BuildPlatform))
+            {
+                throw new Exception("Unrecognized Platform.");
+            }
+
+            foreach (var platform in _buildPlatforms)
+            {
+                if (platform == currentPlatform)
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+    }
+}