Remove dart:io dependency from scanner/ast/parser tests.

pkg/analyzer_experimental/test/generated/scanner_test.dart

 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 library engine.scanner_test;
 import 'package:analyzer_experimental/src/generated/java_core.dart';
-import 'package:analyzer_experimental/src/generated/java_engine_io.dart';
 import 'package:analyzer_experimental/src/generated/java_junit.dart';
 import 'package:analyzer_experimental/src/generated/source.dart';
 import 'package:analyzer_experimental/src/generated/error.dart';
 import 'package:analyzer_experimental/src/generated/scanner.dart';
 import 'package:unittest/unittest.dart' as _ut;
 import 'test_support.dart';
 class KeywordStateTest extends JUnitTestCase {
   void test_KeywordState() {
     List<Keyword> keywords = Keyword.values;
     int keywordCount = keywords.length;
@@ skipping 2125 matching lines @@
   }
   void test_tilde_slash_eq() {
     assertToken(TokenType.TILDE_SLASH_EQ, "~/=");
   }
   void test_unclosedPairInInterpolation() {
     GatheringErrorListener listener = new GatheringErrorListener();
     scan("'\${(}'", listener);
   }
   Token scan(String source, GatheringErrorListener listener);
   void assertComment(TokenType commentType, String source) {
-    Token token = scan2("${source}${OSUtilities.LINE_SEPARATOR}");
+    Token token = scan2("${source}\n");
     JUnitTestCase.assertNotNull(token);
     JUnitTestCase.assertEquals(TokenType.EOF, token.type);
     Token comment = token.precedingComments;
     JUnitTestCase.assertNotNull(comment);
     JUnitTestCase.assertEquals(commentType, comment.type);
     JUnitTestCase.assertEquals(0, comment.offset);
     JUnitTestCase.assertEquals(source.length, comment.length);
     JUnitTestCase.assertEquals(source, comment.lexeme);
   }
 
@@ skipping 125 matching lines @@
     this._lineNumber = lineNumber;
     this._columnNumber = columnNumber;
   }
 }
 main() {
   CharBufferScannerTest.dartSuite();
   KeywordStateTest.dartSuite();
   StringScannerTest.dartSuite();
   TokenTypeTest.dartSuite();
 }