Don't use Set when collecting constant targets in a unit.

pkg/analyzer/test/generated/constant_test.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 analyzer.test.constant_test;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/element.dart';
@@ skipping 604 matching lines @@
     for (ConstantEvaluationTarget target in _findConstants()) {
       if (target is ElementAnnotationImpl) {
         expect(target.context, same(_context));
         expect(target.source, same(_source));
         annotations.add(target.annotationAst);
       }
     }
     return new List<Annotation>.from(annotations);
   }
 
-  Set<ConstantEvaluationTarget> _findConstants() {
+  List<ConstantEvaluationTarget> _findConstants() {
     ConstantFinder finder = new ConstantFinder(_context, _source, _source);
     _node.accept(finder);
-    Set<ConstantEvaluationTarget> constants = finder.constantsToCompute;
+    List<ConstantEvaluationTarget> constants = finder.constantsToCompute;
     expect(constants, isNotNull);
     return constants;
   }
 
   ConstructorElement _setupConstructorDeclaration(String name, bool isConst) {
     Keyword constKeyword = isConst ? Keyword.CONST : null;
     ConstructorDeclaration constructorDeclaration =
         AstFactory.constructorDeclaration2(
             constKeyword,
             null,
@@ skipping 3891 matching lines @@
 
   void _visitNode(AstNode node) {
     node.accept(_createReferenceFinder(_head));
   }
 }
 
 class _TestAnalysisContext extends TestAnalysisContext {
   @override
   InternalAnalysisContext getContextFor(Source source) => this;
 }