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..d3f89ebb97244ada12712c46ceada4ece021239e 100644
--- a/pkg/front_end/test/scanner_test.dart
+++ b/pkg/front_end/test/scanner_test.dart
@@ -1190,6 +1190,7 @@ abstract class ScannerTestBase {
     var openBrace = _scan('{[(') as BeginToken;
     var openBracket = openBrace.next as BeginToken;
     var openParen = openBracket.next as BeginToken;
+    expect(openBrace.previous.type, TokenType.EOF);

[Paul Berry, 2017/02/16 22:07:54]

This seems like a weird place to test this functionality.  How about creating a
new test called something like "test_start_token"?

Also, while we're at it, we should test that the offset of the start token is
correct (currently it's -1 for analyzer's scanner, and 0 for the token stream
from ToAnalyzerTokenStream).

[danrubel, 2017/02/21 14:49:27]

Good points. Found an already existing test_startAndEnd test and enhanced that
one instead.

     expect(openParen.next.type, TokenType.EOF);
     expect(openBrace.endToken, isNull);
     expect(openBracket.endToken, isNull);