Support serialization of the compiler backbone.

tests/compiler/dart2js/analyze_unused_dart2js_test.dart

Index: tests/compiler/dart2js/analyze_unused_dart2js_test.dart
diff --git a/tests/compiler/dart2js/analyze_unused_dart2js_test.dart b/tests/compiler/dart2js/analyze_unused_dart2js_test.dart
index e5c9debffddbe9c606cf5869dd89dc70a8cbb442..f08642865313d15245426461411b099072b578bd 100644
--- a/tests/compiler/dart2js/analyze_unused_dart2js_test.dart
+++ b/tests/compiler/dart2js/analyze_unused_dart2js_test.dart
@@ -38,6 +38,10 @@ const Map<String, List<String>> WHITE_LIST = const {
   "lib/src/constants/expressions.dart": const [
       "The method 'readFromEnvironment' is never called"],
 
+  // Serialization code is only used in test.
+  "lib/src/serialization/": const [
+      "is never"],
+
   // Nested functions are currently kept alive in the IR.
   "lib/src/tree_ir/": const [
     "accept", "FunctionExpression", "CreateFunction"