Throw AbstractClassInstantiationError on abstract classes.

pkg/front_end/lib/src/fasta/loader.dart

 // Copyright (c) 2016, 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.
 
 library fasta.loader;
 
 import 'dart:async' show Future;
 
 import 'dart:collection' show Queue;
 
@@ skipping 121 matching lines @@
       if (library.loader == this) {
         errors.addAll(library.compileTimeErrors);
       }
     }
     return errors;
   }
 
   Builder getCompileTimeError() => target.getCompileTimeError(this);
 
   Builder getNativeAnnotation() => target.getNativeAnnotation(this);
+
+  Builder getAbstractClassInstantiationError() {
+    return target.getAbstractClassInstantiationError(this);
+  }
 }
 
 String format(double d, int fractionDigits, int width) {
   return d.toStringAsFixed(fractionDigits).padLeft(width);
 }