Generalize lookup-maps to support other const keys

tests/compiler/dart2js_extra/lookup_map/reachable_data2_test.dart

Index: tests/compiler/dart2js_extra/lookup_map/reachable_data2_test.dart
diff --git a/tests/compiler/dart2js_extra/lookup_map/reachable_data_test.dart b/tests/compiler/dart2js_extra/lookup_map/reachable_data2_test.dart
similarity index 78%
copy from tests/compiler/dart2js_extra/lookup_map/reachable_data_test.dart
copy to tests/compiler/dart2js_extra/lookup_map/reachable_data2_test.dart
index 8e71ea7ec1899ec5a09fc601916046a40847aa2c..9c824302156be09d3350d56c438de17050f53d92 100644
--- a/tests/compiler/dart2js_extra/lookup_map/reachable_data_test.dart
+++ b/tests/compiler/dart2js_extra/lookup_map/reachable_data2_test.dart
@@ -5,11 +5,12 @@
 import 'package:lookup_map/lookup_map.dart';
 import 'package:expect/expect.dart';
 
-class A{}
-class B{}
-class C{}
-class D{}
-class E{}
+class Key { final x; const Key(this.x); }
+const A = const Key(1);
+const B = const Key(2);
+const C = const Key(3);
+const D = const Key(4);
+const E = const Key(5);
 const map = const LookupMap(const [
     A, const ["the-text-for-A", B],
     B, const ["the-text-for-B", C],