move synthetic fasta closers into the token stream

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

 // Copyright (c) 2011, 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.
 
 library fasta.scanner.token;
 
 import '../../scanner/token.dart' as analyzer;
 
 import 'keyword.dart' show Keyword;
 
@@ skipping 197 matching lines @@
       tail = tail.setNext(token.copy());
       token = token.next;
     }
     return head;
   }
 
   /// Return a copy of the receiver without [preceedingComments].
   Token copyWithoutComments();
 
   @override
-  bool get isSynthetic => charCount == 0;
+  bool get isSynthetic => false;
 
   @override
   analyzer.Keyword get keyword => null;
 
   @override
   bool matchesAny(List<analyzer.TokenType> types) {
     for (analyzer.TokenType type in types) {
       if (this.type == type) {
         return true;
       }
@@ skipping 20 matching lines @@
 
 /**
  * A [SymbolToken] represents the symbol in its precedence info.
  * Also used for end of file with EOF_INFO.
  */
 class SymbolToken extends Token {
   final PrecedenceInfo info;
 
   SymbolToken(this.info, int charOffset) : super(charOffset);
 
-  SymbolToken.eof(int charOffset)
-      : info = EOF_INFO,
-        super(charOffset) {
+  factory SymbolToken.eof(int charOffset) {
+    var eof = new SyntheticSymbolToken(EOF_INFO, charOffset);
     // EOF points to itself so there's always infinite look-ahead.
-    previousToken = this;
-    next = this;
+    eof.previousToken = eof;
+    eof.next = eof;
+    return eof;
   }
 
   String get lexeme => info.value;
 
   String get stringValue => info.value;
 
   bool isIdentifier() => false;
 
   String toString() => "SymbolToken(${info == EOF_INFO ? '-eof-' : lexeme})";
 
   bool get isEof => info == EOF_INFO;
 
   @override
   Token copyWithoutComments() => new SymbolToken(info, charOffset);
 }
 
 /**
+ * A [SyntheticSymbolToken] represents the symbol in its precedence info
+ * which does not exist in the original source.
+ * For example, if the scanner finds '(' missing a ')'
+ * then it will insert an synthetic ')'.
+ */
+class SyntheticSymbolToken extends SymbolToken {
+  SyntheticSymbolToken(PrecedenceInfo info, int charOffset)
+      : super(info, charOffset);
+
+  @override
+  int get charCount => 0;
+
+  @override
+  bool get isSynthetic => true;
+}
+
+/**
  * A [BeginGroupToken] represents a symbol that may be the beginning of
  * a pair of brackets, i.e., ( { [ < or ${
  * The [endGroup] token points to the matching closing bracked in case
  * it can be identified during scanning.
  */
 class BeginGroupToken extends SymbolToken implements analyzer.BeginToken {
   Token endGroup;
 
   BeginGroupToken(PrecedenceInfo info, int charOffset)
       : super(info, charOffset);
@@ skipping 328 matching lines @@
       identical(value, "<=") ||
       identical(value, "<") ||
       identical(value, "&") ||
       identical(value, "^") ||
       identical(value, "|");
 }
 
 bool isTernaryOperator(String value) => identical(value, "[]=");
 
 bool isMinusOperator(String value) => identical(value, "-");