Use ConstExp for storing 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 0c060b79601e47708213f13f0b962d4fc3657763..3e394ad6c10712e9de49266fe2562b814efcc259 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_backend/backend.dart
@@ -409,19 +409,19 @@ class DartConstantTask extends ConstantCompilerTask
 
   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);
     });
@@ -433,13 +433,13 @@ class DartConstantTask extends ConstantCompilerTask
     });
   }
 
-  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(() {