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

compiler/java/com/google/dart/compiler/resolver/Elements.java

Index: compiler/java/com/google/dart/compiler/resolver/Elements.java
diff --git a/compiler/java/com/google/dart/compiler/resolver/Elements.java b/compiler/java/com/google/dart/compiler/resolver/Elements.java
index 01ef8d560cb57458cade19fd36eb7a9183e27e38..34cb6f7f695c6ce61f76abc5053de4bb47aaa316 100644
--- a/compiler/java/com/google/dart/compiler/resolver/Elements.java
+++ b/compiler/java/com/google/dart/compiler/resolver/Elements.java
@@ -713,6 +713,21 @@ static FieldElementImplementation fieldFromNode(DartField node,
     }
     return false;
   }
+  
+  /**
+   * @return <code>true</code> if given {@link Source} represents library with given name.
+   */
+  public static boolean isDart2JsLibrarySource(Source source) {
+    if (source instanceof DartSource) {
+      DartSource dartSource = (DartSource) source;
+      LibrarySource library = dartSource.getLibrary();
+      if (library != null) {
+        String libraryName = library.getName();
+        return libraryName.contains("lib/compiler/implementation/");
+      }
+    }
+    return false;
+  }
 
   /**
    * @return <code>true</code> if given {@link Source} represents code library declaration or