Compute frontend/backend specific constants.

sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
index 34c5c486000834a048a2ef7050cb8013d58d4550..6f90549e6ec3e226bc880f9df9c71193f2138ba1 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
@@ -45,6 +45,14 @@ class DartBackend extends Backend {
   Map<Element, TreeElements> get resolvedElements =>
       compiler.enqueuer.resolution.resolvedElements;
 
+  ConstantSystem get constantSystem {
+    return constantCompiler.constantCompiler.constantSystem;
+  }
+
+  BackendConstantHandler get constantHandler => constantCompiler;
+
+  DartConstantHandler constantCompiler;
+
   /**
    * Tells whether it is safe to remove type declarations from variables,
    * functions parameters. It becomes not safe if:
@@ -96,6 +104,7 @@ class DartBackend extends Backend {
         reexportingLibraries = <Element, LibraryElement>{},
         forceStripTypes = strips.indexOf('types') != -1,
         stripAsserts = strips.indexOf('asserts') != -1,
+        constantCompiler = new DartConstantHandler(compiler),
         super(compiler);
 
   bool classNeedsRti(ClassElement cls) => false;