Move diagnostic from parser to later phase.

src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

@@ -3642,14 +3642,7 @@ private enum PostSkipAction
                 this.ParseAttributeDeclarations(accAttrs);
                 this.ParseModifiers(accMods);
 
-                if (isEvent)
-                {
-                    if (accMods != null && accMods.Count > 0)
-                    {
-                        accMods[0] = this.AddError(accMods[0], ErrorCode.ERR_NoModifiersOnAccessor);
-                    }
-                }
-                else
+                if (!isEvent)
                 {
                     if (accMods != null && accMods.Count > 0)
                     {

src/Compilers/CSharp/Portable/Symbols/Source/SourceCustomEventAccessorSymbol.cs

@@ -85,6 +85,11 @@ internal sealed class SourceCustomEventAccessorSymbol : SourceEventAccessorSymbo
             }
 
             _name = GetOverriddenAccessorName(@event, isAdder) ?? _name;
+
+            if (syntax.Modifiers.Count > 0)
+            {
+                diagnostics.Add(ErrorCode.ERR_NoModifiersOnAccessor, syntax.Modifiers[0].GetLocation());
+            }
         }
 
         internal AccessorDeclarationSyntax GetSyntax()

src/Compilers/CSharp/Test/Syntax/Parsing/ParserErrorMessageTests.cs

@@ -3878,8 +3878,9 @@ public static int Main()
     }
 }
 ";
-
-            ParseAndValidate(test, Diagnostic(ErrorCode.ERR_NoModifiersOnAccessor, "private"), Diagnostic(ErrorCode.ERR_NoModifiersOnAccessor, "public"));
+            CreateCompilationWithMscorlib(test).GetDeclarationDiagnostics().Verify(
+                Diagnostic(ErrorCode.ERR_NoModifiersOnAccessor, "private"), 
+                Diagnostic(ErrorCode.ERR_NoModifiersOnAccessor, "public"));
         }
 
         [Fact]
@@ -3897,7 +3898,9 @@ public class Test
 }
 ";
 
-            ParseAndValidate(test, Diagnostic(ErrorCode.ERR_NoModifiersOnAccessor, "public"), Diagnostic(ErrorCode.ERR_NoModifiersOnAccessor, "private"));
+            CreateCompilationWithMscorlib(test).GetDeclarationDiagnostics().Verify(
+                Diagnostic(ErrorCode.ERR_NoModifiersOnAccessor, "public"),
+                Diagnostic(ErrorCode.ERR_NoModifiersOnAccessor, "private"));
         }
 
         [WorkItem(863423, "DevDiv/Personal")]