fix token type of first token in converted token stream

pkg/front_end/test/scanner_test.dart

Index: pkg/front_end/test/scanner_test.dart
diff --git a/pkg/front_end/test/scanner_test.dart b/pkg/front_end/test/scanner_test.dart
index d853ee87f741c4768ba7904809aa74b39c9c91f2..479c40ec9f92cb3be395576163b7eef75e1b174c 100644
--- a/pkg/front_end/test/scanner_test.dart
+++ b/pkg/front_end/test/scanner_test.dart
@@ -932,12 +932,17 @@ abstract class ScannerTestBase {
 
   void test_startAndEnd() {
     Token token = _scan("a");
+    expect(token.offset, 0);
     Token previous = token.previous;
     expect(previous.next, token);
     expect(previous.previous, previous);
+    expect(previous.type, TokenType.EOF);
+    expect(previous.offset, -1);
     Token next = token.next;
     expect(next.next, next);
     expect(next.previous, token);
+    expect(next.type, TokenType.EOF);
+    expect(next.offset, token.offset + token.length);
   }
 
   void test_string_multi_double() {