Make top-level scope implicit.

pkg/analyzer/lib/src/dart/analysis/file_state.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.
 
 import 'dart:convert';
 import 'dart:typed_data';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/error/listener.dart';
@@ skipping 346 matching lines @@
     if (USE_FASTA_PARSER) {
       try {
         fasta.ScannerResult scanResult =
             fasta.scan(_contentBytes, includeComments: true);
 
         var astBuilder = new fasta.AstBuilder(
             new ErrorReporter(errorListener, source),
             null,
             null,
             new _FastaElementStoreProxy(),
-            new _FastaEmptyScope(),
+            null,
             uri);
         var parser = new fasta.Parser(astBuilder);
         parser.parseUnit(scanResult.tokens);
         var unit = astBuilder.pop() as CompilationUnit;
 
         LineInfo lineInfo = new LineInfo(scanResult.lineStarts);
         unit.lineInfo = lineInfo;
         return unit;
       } catch (e, st) {
         print(e);
@@ skipping 416 matching lines @@
   final _elements = <fasta.Builder, _FastaElementProxy>{};
 
   @override
   _FastaElementProxy operator [](fasta.Builder builder) =>
       _elements.putIfAbsent(builder, () => new _FastaElementProxy());
 
   @override
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }
 
-class _FastaEmptyScope extends fasta.Scope {
-  _FastaEmptyScope() : super({}, null);
-}
-
 class _FastaInterfaceTypeProxy implements fasta.KernelInterfaceType {
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }