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 07f049c188210c7ef7836ea864dabb5f51a7f3f1..db3923a0ab90c271e6626c83c34a8f2bbe299641 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"