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

pkg/analyzer/test/generated/constant_test.dart

Index: pkg/analyzer/test/generated/constant_test.dart
diff --git a/pkg/analyzer/test/generated/constant_test.dart b/pkg/analyzer/test/generated/constant_test.dart
index 4b611b7a817268b1bbe0d3cadf2cfdd722e73070..e5dffdf076f587309ea3b64201bfb279600c1fe3 100644
--- a/pkg/analyzer/test/generated/constant_test.dart
+++ b/pkg/analyzer/test/generated/constant_test.dart
@@ -622,10 +622,10 @@ class ConstantFinderTest {
     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;
   }