Recover from bad supertypes.

pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.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.kernel_field_builder;
 
-import 'package:kernel/ast.dart' show Expression, Field, Library, Name;
+import 'package:kernel/ast.dart' show Expression, Field, Name;
 
 import 'kernel_builder.dart'
-    show Builder, FieldBuilder, KernelTypeBuilder, MetadataBuilder;
+    show
+        Builder,
+        FieldBuilder,
+        KernelTypeBuilder,
+        LibraryBuilder,
+        MetadataBuilder;
 
 class KernelFieldBuilder extends FieldBuilder<Expression> {
   final Field field;
   final List<MetadataBuilder> metadata;
   final KernelTypeBuilder type;
 
   KernelFieldBuilder(this.metadata, this.type, String name, int modifiers,
       Builder compilationUnit, int charOffset)
       : field = new Field(null, fileUri: compilationUnit?.relativeFileUri)
           ..fileOffset = charOffset,
         super(name, modifiers, compilationUnit, charOffset);
 
   void set initializer(Expression value) {
     field.initializer = value..parent = field;
   }
 
-  Field build(Library library) {
-    field.name ??= new Name(name, library);
+  Field build(LibraryBuilder library) {
+    field.name ??= new Name(name, library.target);
     if (type != null) {
-      field.type = type.build();
+      field.type = type.build(library);
     }
     bool isInstanceMember = !isStatic && !isTopLevel;
     return field
       ..isFinal = isFinal
       ..isConst = isConst
       ..hasImplicitGetter = isInstanceMember
       ..hasImplicitSetter = isInstanceMember && !isConst && !isFinal
       ..isStatic = !isInstanceMember;
   }
 
   Field get target => field;
 }