Warn when implicit default value overrides explicit one that differs.

pkg/analyzer/lib/src/generated/resolver.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 engine.resolver;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'java_core.dart';
@@ skipping 1682 matching lines @@
    * @param parameters the list of parameters to be validated
    */
   void _validateDefaultValues(FormalParameterList parameters) {
     if (parameters == null) {
       return;
     }
     for (FormalParameter parameter in parameters.parameters) {
       if (parameter is DefaultFormalParameter) {
         DefaultFormalParameter defaultParameter = parameter;
         Expression defaultValue = defaultParameter.defaultValue;
-        if (defaultValue != null) {
-          DartObjectImpl result = _validate(defaultValue, CompileTimeErrorCode.NON_CONSTANT_DEFAULT_VALUE);
-          VariableElementImpl element = parameter.element as VariableElementImpl;
-          element.evaluationResult = new EvaluationResultImpl.con1(result);
+        DartObjectImpl result;
+        if (defaultValue == null) {
+          result = new DartObjectImpl(_typeProvider.nullType,
+              NullState.NULL_STATE);
+        } else {
+          result = _validate(defaultValue,
+              CompileTimeErrorCode.NON_CONSTANT_DEFAULT_VALUE);
           if (result != null) {
             _reportErrorIfFromDeferredLibrary(defaultValue, CompileTimeErrorCode.NON_CONSTANT_DEFAULT_VALUE_FROM_DEFERRED_LIBRARY);
           }
         }
+        VariableElementImpl element = parameter.element as VariableElementImpl;
+        element.evaluationResult = new EvaluationResultImpl.con1(result);
       }
     }
   }
 
   /**
    * Validates that the expressions of any field initializers in the class declaration are all
    * compile time constants. Since this is only required if the class has a constant constructor,
    * the error is reported at the constructor site.
    *
    * @param classDeclaration the class which should be validated
@@ skipping 13772 matching lines @@
   /**
    * Return the tag that has the given identifier, or {@code null} if there is no such tag (the
    * identifier is not defined).
    *
    * @return the tag that has the given identifier
    */
   String getTagWithId(String identifier) {
     return idToTagMap[identifier];
   }
 }