Better error handling when scanning library tags

runtime/vm/scanner.h

 // 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.
 //
 // Scanner class for the Dart language. The scanner reads source text
 // and produces a stream of tokens which is used by the parser.
 //
 
 #ifndef VM_SCANNER_H_
 #define VM_SCANNER_H_
@@ skipping 82 matching lines @@
     int keyword_len;
     String* keyword_symbol;
   };
 
   // Initialize Scanner tables.
   void InitKeywordTable();
 
   // Reads next lookahead character.
   void ReadChar();
 
+  // Read and discard characters up to end of line.
+  void SkipLine();
+
   // Recognizes token 'kind' and reads next character in input.
   void Recognize(Token::Kind kind) {
     ReadChar();
     current_token_.kind = kind;
   }
 
   int32_t LookaheadChar(int how_many);
 
   void ErrorMsg(const char* msg);
 
@@ skipping 78 matching lines @@
   const String& private_key_;
 
   SourcePosition c0_pos_;      // Source position of lookahead character c0_.
   KeywordTable keywords_[Token::numKeywords];
 };
 
 
 }  // namespace dart
 
 #endif  // VM_SCANNER_H_