Change to a coreclr-portable Encoding API

Compilers.sln

@@ -104,7 +104,6 @@ EndProject
 Global
 	GlobalSection(SharedMSBuildProjectFiles) = preSolution
 		src\Test\Utilities\Shared\TestUtilities.projitems*{6ff42825-5464-4151-ac55-ed828168c192}*SharedItemsImports = 13
-		src\Test\Utilities\Shared\TestUtilities.projitems*{f7712928-1175-47b3-8819-ee086753dee2}*SharedItemsImports = 4
 		src\Compilers\Core\AnalyzerDriver\AnalyzerDriver.projitems*{d0bc9be7-24f6-40ca-8dc6-fcb93bd44b34}*SharedItemsImports = 13
 		src\Compilers\Core\SharedCollections\SharedCollections.projitems*{afde6bea-5038-4a4a-a88e-dbd2e4088eed}*SharedItemsImports = 4
 		src\Compilers\Core\SharedCollections\SharedCollections.projitems*{1ee8cad3-55f9-4d91-96b2-084641da9a6c}*SharedItemsImports = 4

src/Test/Utilities/Shared/Assert/ConditionalFactAttribute.cs

@@ -33,7 +33,7 @@ public class x86 : ExecutionCondition
 
     public class HasShiftJisDefaultEncoding : ExecutionCondition
     {
-        public override bool ShouldSkip => Encoding.GetEncoding(0)?.WindowsCodePage != 932;
+        public override bool ShouldSkip => Encoding.GetEncoding(0)?.CodePage != 932;
 
         public override string SkipReason => "OS default codepage is not Shift-JIS (932).";
     }