Using 'interface' and 'abstract' for methods should produce error

pkg/dartdoc/lib/mirrors.dart

Index: pkg/dartdoc/lib/mirrors.dart
diff --git a/pkg/dartdoc/lib/mirrors.dart b/pkg/dartdoc/lib/mirrors.dart
index d6a8010834419263bee8c0ece2f097ec1b0f48f8..82d48429e8d5d214ce6489006a1200800748df94 100644
--- a/pkg/dartdoc/lib/mirrors.dart
+++ b/pkg/dartdoc/lib/mirrors.dart
@@ -13,7 +13,7 @@
 /**
  * [Compilation] encapsulates the compilation of a program.
  */
-class Compilation {
+abstract class Compilation {
   /**
    * Creates a new compilation which has [script] as its entry point.
    */
@@ -45,7 +45,7 @@ class Compilation {
   /**
    * Returns a future for the compiled JavaScript code.
    */
-  abstract Future<String> compileToJavaScript();
+  Future<String> compileToJavaScript();
 }
 
 /**