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

pkg/analyzer/lib/src/generated/constant.dart

Index: pkg/analyzer/lib/src/generated/constant.dart
diff --git a/pkg/analyzer/lib/src/generated/constant.dart b/pkg/analyzer/lib/src/generated/constant.dart
index 71230291bd61c840ac741c656850e037d8795334..7a02fa3610c6416fd0adf8d5d192d815dbb28c52 100644
--- a/pkg/analyzer/lib/src/generated/constant.dart
+++ b/pkg/analyzer/lib/src/generated/constant.dart
@@ -1248,8 +1248,8 @@ class ConstantFinder extends RecursiveAstVisitor<Object> {
   /**
    * 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".