Use identical to compare strings.

pkg/front_end/lib/src/fasta/parser/parser.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.
 
 library fasta.parser.parser;
 
 import '../fasta_codes.dart'
     show
         FastaCode,
         FastaMessage,
@@ skipping 1007 matching lines @@
       listener.handleNoType(token);
     }
     listener.endTypeVariable(token, extendsOrSuper);
     return token;
   }
 
   /// Returns true if the stringValue of the [token] is either [value1],
   /// [value2], or [value3].
   bool isOneOf3(Token token, String value1, String value2, String value3) {
     String stringValue = token.stringValue;
-    return value1 == stringValue ||
-        value2 == stringValue ||
-        value3 == stringValue;
+    return identical(value1, stringValue) ||
+        identical(value2, stringValue) ||
+        identical(value3, stringValue);
   }
 
   /// Returns true if the stringValue of the [token] is either [value1],
   /// [value2], [value3], or [value4].
   bool isOneOf4(
       Token token, String value1, String value2, String value3, String value4) {
     String stringValue = token.stringValue;
-    return value1 == stringValue ||
-        value2 == stringValue ||
-        value3 == stringValue ||
-        value4 == stringValue;
+    return identical(value1, stringValue) ||
+        identical(value2, stringValue) ||
+        identical(value3, stringValue) ||
+        identical(value4, stringValue);
   }
 
   bool notEofOrValue(String value, Token token) {
     return !identical(token.kind, EOF_TOKEN) &&
         !identical(value, token.stringValue);
   }
 
   bool isGeneralizedFunctionType(Token token) {
     return optional('Function', token) &&
         (optional('<', token.next) || optional('(', token.next));
@@ skipping 2803 matching lines @@
     previous.setNext(firstToken);
     beforeToken = firstToken;
   }
 }
 
 typedef FastaMessage NoArgument(Uri uri, int charOffset);
 
 typedef FastaMessage TokenArgument(Uri uri, int charOffset, Token token);
 
 typedef FastaMessage StringArgument(Uri uri, int charOffset, String string);