Prettify !(x rel-op y) into (x "!rel-op" y) when possible.

lib/compiler/implementation/operations.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 Operation {
   final SourceString name;
   bool isUserDefinable();
 }
 
 interface UnaryOperation extends Operation {
@@ skipping 150 matching lines @@
       }
       // A division by 0 means that we might not have a folded value.
       if (foldedValue === null) return null;
       if (left.isInt() && right.isInt() && !isDivide()) {
         assert(foldedValue is int);
         return new IntConstant(foldedValue);
       } else {
         return new DoubleConstant(foldedValue);
       }
     }
+    return null;
   }
 
   bool isDivide() => false;
   num foldInts(int left, int right) => foldNums(left, right);
   abstract num foldNums(num left, num right);
 }
 
 class SubtractOperation extends ArithmeticNumOperation {
   final SourceString name = const SourceString('-');
   const SubtractOperation();
@@ skipping 131 matching lines @@
   bool isUserDefinable() => false;
   const IdentityOperation();
   Constant fold(Constant left, Constant right) {
     // In order to preserve runtime semantics which says that NaN !== NaN don't
     // constant fold NaN === NaN. Otherwise the output depends on inlined
     // variables and other optimizations.
     if (left.isNaN() && right.isNaN()) return null;
     return new BoolConstant(left == right);
   }
 }