modifiy insertTokenBefore to return the inserted token

pkg/front_end/test/fasta/parser/token_stream_rewriter_test.dart

 // Copyright (c) 2017, 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 'package:front_end/src/fasta/parser/token_stream_rewriter.dart';
 import 'package:front_end/src/fasta/scanner/token.dart';
 import 'package:front_end/src/scanner/token.dart'
     show BeginToken, Token, TokenType;
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(TokenStreamRewriterTest_NoPrevious);
     defineReflectiveTests(TokenStreamRewriterTest_UsingPrevious);
   });
 }
 
 /// Abstract base class for tests of [TokenStreamRewriter].
 abstract class TokenStreamRewriterTest {
   /// Indicates whether the tests should set up [Token.previous].
   bool get setPrevious;
 
   void test_insert_at_end() {
     var a = _makeToken(0, 'a');
     var b = _makeToken(1, 'b');
     var eof = _link([a]);
     var rewriter = new TokenStreamRewriter(a);
-    rewriter.insertTokenBefore(b, eof);
+    expect(rewriter.insertTokenBefore(b, eof), same(b));
     expect(rewriter.firstToken, same(a));
     expect(a.next, same(b));
     expect(b.next, same(eof));
     expect(eof.previous, same(b));
     expect(b.previous, same(a));
   }
 
   void test_insert_at_start() {
     var a = _makeToken(0, 'a');
     var b = _makeToken(1, 'b');
     _link([b]);
     var rewriter = new TokenStreamRewriter(b);
-    rewriter.insertTokenBefore(a, b);
+    expect(rewriter.insertTokenBefore(a, b), same(a));
     expect(rewriter.firstToken, same(a));
     expect(a.next, same(b));
     expect(a.previous.next, same(a));
     expect(b.previous, same(a));
   }
 
   void test_resume_at_previous_insertion_point() {
     var a = _makeToken(0, 'a');
     var b = _makeToken(1, 'b');
     var c = _makeToken(2, 'c');
@@ skipping 90 matching lines @@
 /// Concrete implementation of [TokenStreamRewriterTest] in which
 /// [Token.previous] values are set to non-null.
 ///
 /// Since [TokenStreamRewriter] makes use of [Token.previous] when it can,
 /// these tests do not exercise the more complex heuristics for finding previous
 /// tokens.
 @reflectiveTest
 class TokenStreamRewriterTest_UsingPrevious extends TokenStreamRewriterTest {
   bool get setPrevious => true;
 }