fasta scanner recovery and error code translation improvements

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

Index: pkg/front_end/lib/src/fasta/scanner/token.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/token.dart b/pkg/front_end/lib/src/fasta/scanner/token.dart
index 35067a2e04cce24eb47ab156400ef5d2820181a5..6e3ffae8557e3cdeb94d4c20770ab43bc976f1b6 100644
--- a/pkg/front_end/lib/src/fasta/scanner/token.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/token.dart
@@ -6,6 +6,8 @@ library fasta.scanner.token;
 
 import '../../scanner/token.dart' as analyzer;
 
+import '../errors.dart' show internalError;
+
 import 'keyword.dart' show Keyword;
 
 import 'precedence.dart'
@@ -16,8 +18,6 @@ import 'precedence.dart'
         IDENTIFIER_INFO,
         IS_INFO,
         KEYWORD_INFO,
-        MULTI_LINE_COMMENT_INFO,
-        SINGLE_LINE_COMMENT_INFO,
         PrecedenceInfo;
 
 import 'token_constants.dart' show IDENTIFIER_TOKEN;
@@ -258,13 +258,21 @@ class SymbolToken extends Token {
 
   SymbolToken(this.info, int charOffset) : super(charOffset);
 
+  SymbolToken.eof(int charOffset)
+      : info = EOF_INFO,
+        super(charOffset) {
+    // EOF points to itself so there's always infinite look-ahead.
+    previousToken = this;
+    next = this;
+  }
+
   String get lexeme => info.value;
 
   String get stringValue => info.value;
 
   bool isIdentifier() => false;
 
-  String toString() => "SymbolToken($lexeme)";
+  String toString() => "SymbolToken(${info == EOF_INFO ? '-eof-' : lexeme})";
 
   bool get isEof => info == EOF_INFO;
 
@@ -490,13 +498,13 @@ class CommentToken extends StringToken implements analyzer.CommentToken {
 
   @override
   void set parent(analyzer.TokenWithComment ignored) {
-    throw 'unsupported operation';
+    internalError('Internal error: unsupported operation');
   }
 
   @override
   void remove() {
-    // TODO: implement remove
-    throw 'not implemented yet';
+    // TODO(danrubel): implement remove
+    internalError('Internal error: not implemented yet');
   }
 }