Validate that int values that we write to summaries are >= 0.

pkg/analyzer/test/generated/resolver_test.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.test.generated.resolver_test;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ skipping 109 matching lines @@
     // dart:core
     //
     TestTypeProvider provider = new TestTypeProvider();
     CompilationUnitElementImpl coreUnit =
         new CompilationUnitElementImpl("core.dart");
     Source coreSource = sourceFactory.forUri(DartSdk.DART_CORE);
     coreContext.setContents(coreSource, "");
     coreUnit.librarySource = coreUnit.source = coreSource;
     ClassElementImpl proxyClassElement = ElementFactory.classElement2("_Proxy");
     proxyClassElement.constructors = <ConstructorElement>[
-      ElementFactory.constructorElement(proxyClassElement, null, true)
+      ElementFactory.constructorElement(proxyClassElement, '', true)
         ..isCycleFree = true
         ..constantInitializers = <ConstructorInitializer>[]
     ];
     ClassElement objectClassElement = provider.objectType.element;
     coreUnit.types = <ClassElement>[
       provider.boolType.element,
       provider.deprecatedType.element,
       provider.doubleType.element,
       provider.functionType.element,
       provider.intType.element,
@@ skipping 16762 matching lines @@
 
   void _resolveTestUnit(String code) {
     testCode = code;
     testSource = addSource(testCode);
     LibraryElement library = resolve2(testSource);
     assertNoErrors(testSource);
     verify([testSource]);
     testUnit = resolveCompilationUnit(testSource, library);
   }
 }