Generate needed classes for constant maps in metadata.

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

Index: sdk/lib/_internal/compiler/implementation/compiler.dart
diff --git a/sdk/lib/_internal/compiler/implementation/compiler.dart b/sdk/lib/_internal/compiler/implementation/compiler.dart
index 20b914c05f6db63df2f9b5d1187ed49e46c3d76e..313fbd07495a430a28dabbf7a718cd84b3490c03 100644
--- a/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -278,9 +278,9 @@ abstract class Backend {
   /// Called during codegen when [constant] has been used.
   void registerCompileTimeConstant(Constant constant, Registry registry) {}
 
-  /// Called during resolution when a metadata [constant] for [annotatedElement]
-  /// has been evaluated.
-  void registerMetadataConstant(Constant constant,
+  /// Called during resolution when a constant value for [metadata] on
+  /// [annotatedElement] has been evaluated.
+  void registerMetadataConstant(MetadataAnnotation metadata,
                                 Element annotatedElement,
                                 Registry registry) {}