Make an empty identifier a scanning error.

lib/compiler/implementation/scanner/scanner.dart

 // Copyright (c) 2012, 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.
 
 interface Scanner {
   Token tokenize();
 }
 
 /**
  * Common base class for a Dart scanner.
@@ skipping 635 matching lines @@
   int tokenizeIdentifier(int next, int start, bool allowDollar) {
     bool isAscii = true;
     while (true) {
       if (($a <= next && next <= $z) ||
           ($A <= next && next <= $Z) ||
           ($0 <= next && next <= $9) ||
           next === $_ ||
           (next === $$ && allowDollar)) {
         next = advance();
       } else if (next < 128) {
+        // Identifier ends here.
+        if (start == byteOffset) {
+          throw new MalformedInputException("expected identifier not found",
+                                            charOffset);
+        }
         if (isAscii) {
           appendByteStringToken(IDENTIFIER_INFO, asciiString(start, 0));
         } else {
           appendByteStringToken(IDENTIFIER_INFO, utf8String(start, -1));
         }
         return next;
       } else {
         int nonAsciiStart = byteOffset;
         do {
           next = nextByte();
@@ skipping 161 matching lines @@
                                       charOffset);
   }
 }
 
 class MalformedInputException {
   final String message;
   final position;
   MalformedInputException(this.message, this.position);
   toString() => message;
 }