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

pkg/analyzer/lib/src/generated/constant.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.src.generated.constant;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ skipping 1230 matching lines @@
  * those compilation units.
  */
 class ConstantFinder extends RecursiveAstVisitor<Object> {
   final AnalysisContext context;
   final Source source;
   final Source librarySource;
 
   /**
    * The elements and AST nodes whose constant values need to be computed.
    */
-  HashSet<ConstantEvaluationTarget> constantsToCompute =
-      new HashSet<ConstantEvaluationTarget>();
+  List<ConstantEvaluationTarget> constantsToCompute =
+      <ConstantEvaluationTarget>[];
 
   /**
    * True if instance variables marked as "final" should be treated as "const".
    */
   bool treatFinalInstanceVarAsConst = false;
 
   ConstantFinder(this.context, this.source, this.librarySource);
 
   @override
   Object visitAnnotation(Annotation node) {
@@ skipping 4149 matching lines @@
       return BoolState.from(_element == rightElement);
     } else if (rightOperand is DynamicState) {
       return BoolState.UNKNOWN_VALUE;
     }
     return BoolState.FALSE_STATE;
   }
 
   @override
   String toString() => _element == null ? "-unknown-" : _element.name;
 }