A missing enum case in a switch is a warning, not a compile-time error.

pkg/analyzer/lib/src/generated/error_verifier.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.generated.error_verifier;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 4262 matching lines @@
       }
     }
     int nameCount = constantNames.length;
     if (nameCount == 0) {
       return false;
     }
     for (int i = 0; i < nameCount; i++) {
       int offset = statement.offset;
       int end = statement.rightParenthesis.end;
       _errorReporter.reportErrorForOffset(
-          CompileTimeErrorCode.MISSING_ENUM_CONSTANT_IN_SWITCH,
+          StaticWarningCode.MISSING_ENUM_CONSTANT_IN_SWITCH,
           offset,
           end - offset,
           [constantNames[i]]);
     }
     return true;
   }
 
   /**
    * Verify that the given function [body] does not contain return statements
    * that both have and do not have return values.
@@ skipping 1909 matching lines @@
     toCheck.add(type.element);
     // type arguments
     if (type is InterfaceType) {
       InterfaceType interfaceType = type;
       for (DartType typeArgument in interfaceType.typeArguments) {
         _addTypeToCheck(typeArgument);
       }
     }
   }
 }