Pass along extern

src/Workspaces/CSharp/Portable/CodeGeneration/CSharpSyntaxGenerator.cs

@@ -1487,7 +1487,11 @@ public override SyntaxNode WithAccessibility(SyntaxNode declaration, Accessibili
 
         private static readonly DeclarationModifiers s_interfaceModifiers = DeclarationModifiers.New | DeclarationModifiers.Partial | DeclarationModifiers.Unsafe;
         private static readonly DeclarationModifiers s_accessorModifiers = DeclarationModifiers.Abstract | DeclarationModifiers.New | DeclarationModifiers.Override | DeclarationModifiers.Virtual;
-        private static readonly DeclarationModifiers s_localFunctionModifiers = DeclarationModifiers.Async | DeclarationModifiers.Static;
+
+        private static readonly DeclarationModifiers s_localFunctionModifiers =
+            DeclarationModifiers.Async |
+            DeclarationModifiers.Static |
+            DeclarationModifiers.Extern;
 
         private static DeclarationModifiers GetAllowedModifiers(SyntaxKind kind)
         {

src/Workspaces/Core/Portable/Editing/DeclarationModifiers.cs

@@ -65,14 +65,14 @@ public static DeclarationModifiers From(ISymbol symbol)
             return new DeclarationModifiers(
                 isStatic: symbol.IsStatic,
                 isAbstract: symbol.IsAbstract,
-                ////isNew: (property != null && property.OverriddenProperty == null) || (method != null && method.OverriddenMethod == null),
-                isReadOnly: (field != null && field.IsReadOnly) || (property != null && property.IsReadOnly),
+                isReadOnly: field?.IsReadOnly == true || property?.IsReadOnly == true,
                 isVirtual: symbol.IsVirtual,
                 isOverride: symbol.IsOverride,
                 isSealed: symbol.IsSealed,
                 isConst: field != null && field.IsConst,
                 isUnsafe: symbol.IsUnsafe(),
-                isVolatile: field != null && field.IsVolatile);
+                isVolatile: field != null && field.IsVolatile,
+                isExtern: symbol.IsExtern);
         }
 
         public bool IsStatic => (_modifiers & Modifiers.Static) != 0;