Bring Host build into separate project

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

@@ -0,0 +1,34 @@
+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(nameof(platforms));
+            }
+
+            _buildPlatforms = platforms;
+        }
+
+        public override bool EvaluateCondition()
+        {
+            foreach (var platform in _buildPlatforms)
+            {
+                if (CurrentPlatform.IsPlatform(platform))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+    }
+}