Fix `overriden_field` false positive (#246).

lib/src/rules/overriden_field.dart

 // Copyright (c) 2016, 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 linter.src.rules.overriden_field;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ skipping 95 matching lines @@
     node.fields.variables.forEach((VariableDeclaration variable) {
       PropertyAccessorElement field = _getOverriddenMember(variable.element);
       if (field != null) {
         rule.reportLint(variable.name);
       }
     });
   }
 
   PropertyAccessorElement _getOverriddenMember(Element member) {
     String memberName = member.name;
+    LibraryElement library = member.library;
     bool isOverriddenMember(PropertyAccessorElement a) =>
-        a.isSynthetic && a.name == memberName;
+        a.library == library && a.isSynthetic && a.name == memberName;
     bool containsOverridenMember(InterfaceType i) =>
         i.accessors.any(isOverriddenMember);
     ClassElement classElement = member.enclosingElement;
 
     Iterable<InterfaceType> interfaces =
         _findAllSupertypesAndMixins(classElement.type, <InterfaceType>[]);
     InterfaceType interface =
         interfaces.firstWhere(containsOverridenMember, orElse: () => null);
     return interface == null
         ? null
         : interface.accessors.firstWhere(isOverriddenMember);
   }
 }