Support serialization of the compiler backbone.

pkg/compiler/lib/src/constants/expressions.dart

Index: pkg/compiler/lib/src/constants/expressions.dart
diff --git a/pkg/compiler/lib/src/constants/expressions.dart b/pkg/compiler/lib/src/constants/expressions.dart
index dc0a3d9c811489aa12f3f1700c6d5afd758d8702..4b1060c677ed0e4b6bcd90291a45f3bcde81e122 100644
--- a/pkg/compiler/lib/src/constants/expressions.dart
+++ b/pkg/compiler/lib/src/constants/expressions.dart
@@ -108,6 +108,22 @@ abstract class ConstantConstructor {
   Map<FieldElement, ConstantExpression> computeInstanceFields(
       List<ConstantExpression> arguments,
       CallStructure callStructure);
+
+  accept(ConstantConstructorVisitor visitor, arg);
+}
+
+abstract class ConstantConstructorVisitor<R, A> {
+  const ConstantConstructorVisitor();
+
+  R visit(ConstantConstructor constantConstructor, A context) {
+    return constantConstructor.accept(this, context);
+  }
+
+  R visitGenerative(GenerativeConstantConstructor constructor, A arg);
+  R visitRedirectingGenerative(
+      RedirectingGenerativeConstantConstructor constructor, A arg);
+  R visitRedirectingFactory(
+      RedirectingFactoryConstantConstructor constructor, A arg);
 }
 
 /// A generative constant constructor.
@@ -142,6 +158,10 @@ class GenerativeConstantConstructor implements ConstantConstructor{
     return appliedFieldMap;
   }
 
+  accept(ConstantConstructorVisitor visitor, arg) {
+    return visitor.visitGenerative(this, arg);
+  }
+
   int get hashCode {
     int hash = Hashing.objectHash(type);
     hash = Hashing.mapHash(defaultValues, hash);
@@ -233,6 +253,10 @@ class RedirectingGenerativeConstantConstructor implements ConstantConstructor {
     return appliedFieldMap;
   }
 
+  accept(ConstantConstructorVisitor visitor, arg) {
+    return visitor.visitRedirectingGenerative(this, arg);
+  }
+
   int get hashCode {
     int hash = Hashing.objectHash(thisConstructorInvocation);
     return Hashing.mapHash(defaultValues, hash);
@@ -282,6 +306,10 @@ class RedirectingFactoryConstantConstructor implements ConstantConstructor {
     return constantConstructor.computeInstanceFields(arguments, callStructure);
   }
 
+  accept(ConstantConstructorVisitor visitor, arg) {
+    return visitor.visitRedirectingFactory(this, arg);
+  }
+
   int get hashCode {
     return Hashing.objectHash(targetConstructorInvocation);
   }