Add support for metadata annotation syntax (issue 4056)

compiler/java/com/google/dart/compiler/parser/DartScanner.java

 // 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.
 
 package com.google.dart.compiler.parser;
 
 import com.google.dart.compiler.metrics.DartEventType;
 import com.google.dart.compiler.metrics.Tracer;
 import com.google.dart.compiler.metrics.Tracer.TraceEvent;
 import com.google.dart.compiler.parser.DartScanner.InternalState.Mode;
@@ skipping 1190 matching lines @@
           return Token.BIT_NOT;
         }
 
       case '@':
         // Raw strings.
         advance();
         if (is('\'') || is('"')) {
           boolean isRaw = true;
           return scanString(isRaw);
         } else {
-          return select(Token.ILLEGAL);
+          return Token.AT;
         }
 
       case '#':
         return scanDirective();
 
       default:
         if (isIdentifierStart(lookahead(0))) {
           boolean allowDollars = true;
           return scanIdentifier(allowDollars);
         }
@@ skipping 109 matching lines @@
     while (true) {
       int c = lookahead(0);
       if (isLineTerminator(c)) {
       } else if (!isWhiteSpace(c)) {
         break;
       }
       advance();
     }
   }
 }