move synthetic fasta closers into the token stream

pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart

Index: pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
index f98242cda29804f83b275aafc50dd5767b6e0a78..5e4f34518739517d5d057bd244bcaad369932c36 100644
--- a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
@@ -11,7 +11,7 @@ import 'dart:typed_data' show Uint16List, Uint32List;
 import '../scanner.dart'
     show ErrorToken, Scanner, buildUnexpectedCharacterToken;
 
-import 'error_token.dart' show UnmatchedToken, UnterminatedToken;
+import 'error_token.dart' show UnterminatedToken;
 
 import 'keyword.dart' show KeywordState, Keyword;
 
@@ -1116,57 +1116,6 @@ abstract class AbstractScanner implements Scanner {
       return -1;
     }
   }
-
-  void unmatchedBeginGroup(BeginGroupToken begin) {
-    // We want to ensure that unmatched BeginGroupTokens are reported as
-    // errors.  However, the diet parser assumes that groups are well-balanced
-    // and will never look at the endGroup token.  This is a nice property that
-    // allows us to skip quickly over correct code. By inserting an additional
-    // synthetic token in the stream, we can keep ignoring endGroup tokens.
-    //
-    // [begin] --next--> [tail]
-    // [begin] --endG--> [synthetic] --next--> [next] --next--> [tail]
-    //
-    // This allows the diet parser to skip from [begin] via endGroup to
-    // [synthetic] and ignore the [synthetic] token (assuming it's correct),
-    // then the error will be reported when parsing the [next] token.
-    //
-    // For example, tokenize("{[1};") produces:
-    //
-    // SymbolToken({) --endGroup-----+
-    //      |                        |
-    //     next                      |
-    //      v                        |
-    // SymbolToken([) --endGroup--+  |
-    //      |                     |  |
-    //     next                   |  |
-    //      v                     |  |
-    // StringToken(1)             |  |
-    //      |                     v  |
-    //     next       SymbolToken(]) | <- Synthetic token.
-    //      |                     |  |
-    //      |                   next |
-    //      v                     |  |
-    // UnmatchedToken([)<---------+  |
-    //      |                        |
-    //     next                      |
-    //      v                        |
-    // SymbolToken(})<---------------+
-    //      |
-    //     next
-    //      v
-    // SymbolToken(;)
-    //      |
-    //     next
-    //      v
-    //     EOF
-    Token synthetic =
-        new SymbolToken(closeBraceInfoFor(begin), begin.charOffset);
-    UnmatchedToken next = new UnmatchedToken(begin);
-    begin.endGroup = synthetic;
-    synthetic.next = next;
-    appendErrorToken(next);
-  }
 }
 
 PrecedenceInfo closeBraceInfoFor(BeginGroupToken begin) {