Support partial methods properly.

src/EditorFeatures/CSharpTest/AddAccessibilityModifiers/AddAccessibilityModifiersTests.cs

@@ -50,6 +50,7 @@ struct NestedStruct { }
             void M1() { }
             public void M2() { }
             void I.M3() { }
+            partial void M4() { }
 
             int P1 { get; }
             public int P2 { get; }
@@ -107,6 +108,7 @@ private struct NestedStruct { }
             private void M1() { }
             public void M2() { }
             void I.M3() { }
+            partial void M4() { }
 
             private int P1 { get; }
             public int P2 { get; }

src/EditorFeatures/VisualBasicTest/AddAccessibilityModifiers/AddAccessibilityModifiersTests.vb

@@ -61,6 +61,9 @@ namespace N
             public function M5() as integer
             end function
 
+            partial sub M6()
+            end sub
+
             property P1 as integer
             
             property P2 as integer
@@ -150,6 +153,9 @@ namespace N
             public function M5() as integer
             end function
 
+            Public partial sub M6()
+            end sub
+
             Public property P1 as integer
             
             Public property P2 as integer

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

@@ -1444,7 +1444,20 @@ internal override bool CanHaveAccessibility(SyntaxNode declaration)
                     return ((IndexerDeclarationSyntax)declaration).ExplicitInterfaceSpecifier == null;
 
                 case SyntaxKind.MethodDeclaration:
-                    return ((MethodDeclarationSyntax)declaration).ExplicitInterfaceSpecifier == null;
+                    var method = (MethodDeclarationSyntax)declaration;
+                    if (method.ExplicitInterfaceSpecifier != null)
+                    {
+                        // explicit interface methods can't have accessibility.
+                        return false;
+                    }
+
+                    if (method.Modifiers.Any(SyntaxKind.PartialKeyword))
+                    {
+                        // partial methods can't have accessibility modifiers.
+                        return false;
+                    }
+
+                    return true;
 
                 case SyntaxKind.EventDeclaration:
                     return ((EventDeclarationSyntax)declaration).ExplicitInterfaceSpecifier == null;