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

lib/compiler/implementation/compile_time_constants.dart

Index: lib/compiler/implementation/compile_time_constants.dart
diff --git a/lib/compiler/implementation/compile_time_constants.dart b/lib/compiler/implementation/compile_time_constants.dart
index c035a937b1c8aa508ef51281d1735a71360dc21a..012b236f00b6bc4ad6634782c19e823dec497e47 100644
--- a/lib/compiler/implementation/compile_time_constants.dart
+++ b/lib/compiler/implementation/compile_time_constants.dart
@@ -79,7 +79,7 @@ class NumConstant extends PrimitiveConstant {
 class IntConstant extends NumConstant {
   final int value;
   factory IntConstant(int value) {
-    switch(value) {
+    switch (value) {
       case 0: return const IntConstant._internal(0);
       case 1: return const IntConstant._internal(1);
       case 2: return const IntConstant._internal(2);
@@ -677,16 +677,15 @@ class ConstantHandler extends CompilerTask {
         } else if (code >= 0x80) {
           if (code < 0x100) {
             buffer.add(@'\x');
-            buffer.add(code.toRadixString(16));
           } else {
             buffer.add(@'\u');
             if (code < 0x1000) {
               buffer.add('0');
             }
-            buffer.add(code.toRadixString(16));
           }
+          buffer.add(code.toRadixString(16));
         } else {
-          buffer.add(new String.fromCharCodes(<int>[code]));
+          buffer.addCharCode(code);
         }
       }
     }