allow dart:* libs to implement otherwise disallowed types (e.g. int)

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 engine.resolver.error_verifier;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/src/generated/static_type_analyzer.dart';
@@ skipping 3181 matching lines @@
    * [_checkForAllMixinErrorCodes],
    * [CompileTimeErrorCode.EXTENDS_DISALLOWED_CLASS],
    * [CompileTimeErrorCode.IMPLEMENTS_DISALLOWED_CLASS], and
    * [CompileTimeErrorCode.MIXIN_OF_DISALLOWED_CLASS].
    */
   bool _checkForExtendsOrImplementsDisallowedClass(
       TypeName typeName, ErrorCode errorCode) {
     if (typeName.isSynthetic) {
       return false;
     }
+    // The SDK implementation may implement disallowed types. For example,
+    // JSNumber in dart2js and _Smi in Dart VM both implement int.
+    if (_currentLibrary.source.isInSystemLibrary) {
+      return false;
+    }
     DartType superType = typeName.type;
     for (InterfaceType disallowedType
         in _DISALLOWED_TYPES_TO_EXTEND_OR_IMPLEMENT) {
       if (superType != null && superType == disallowedType) {
         // if the violating type happens to be 'num', we need to rule out the
         // case where the enclosing class is 'int' or 'double'
         if (superType == _typeProvider.numType) {
           AstNode grandParent = typeName.parent.parent;
           // Note: this is a corner case that won't happen often, so adding a
           // field currentClass (see currentFunction) to ErrorVerifier isn't
@@ skipping 2859 matching lines @@
     toCheck.add(type.element);
     // type arguments
     if (type is InterfaceType) {
       InterfaceType interfaceType = type;
       for (DartType typeArgument in interfaceType.typeArguments) {
         _addTypeToCheck(typeArgument);
       }
     }
   }
 }