Use ConstExp for storing constants.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 391 matching lines @@
 class DartConstantTask extends ConstantCompilerTask
     implements BackendConstantEnvironment {
   final DartConstantCompiler constantCompiler;
 
   DartConstantTask(Compiler compiler)
     : this.constantCompiler = new DartConstantCompiler(compiler),
       super(compiler);
 
   String get name => 'ConstantHandler';
 
-  Constant getConstantForVariable(VariableElement element) {
+  ConstExp getConstantForVariable(VariableElement element) {
     return constantCompiler.getConstantForVariable(element);
   }
 
-  Constant getConstantForNode(Node node, TreeElements elements) {
+  ConstExp getConstantForNode(Node node, TreeElements elements) {
     return constantCompiler.getConstantForNode(node, elements);
   }
 
-  Constant getConstantForMetadata(MetadataAnnotation metadata) {
-    return metadata.value;
+  ConstExp getConstantForMetadata(MetadataAnnotation metadata) {
+    return metadata.constant;
   }
 
-  Constant compileConstant(VariableElement element) {
+  ConstExp compileConstant(VariableElement element) {
     return measure(() {
       return constantCompiler.compileConstant(element);
     });
   }
 
   void compileVariable(VariableElement element) {
     measure(() {
       constantCompiler.compileVariable(element);
     });
   }
 
-  Constant compileNode(Node node, TreeElements elements) {
+  ConstExp compileNode(Node node, TreeElements elements) {
     return measure(() {
       return constantCompiler.compileNodeWithDefinitions(node, elements);
     });
   }
 
-  Constant compileMetadata(MetadataAnnotation metadata,
+  ConstExp compileMetadata(MetadataAnnotation metadata,
                            Node node,
                            TreeElements elements) {
     return measure(() {
       return constantCompiler.compileMetadata(metadata, node, elements);
     });
   }
 }