js_ast: Better escaping of strings in preparation for utf8 file encoding

pkg/compiler/lib/src/js_backend/constant_emitter.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.
 
 part of js_backend;
 
 typedef jsAst.Expression _ConstantReferenceGenerator(ConstantValue constant);
 
 typedef jsAst.Expression _ConstantListGenerator(jsAst.Expression array);
 
@@ skipping 135 matching lines @@
     }
   }
 
   /**
    * Write the contents of the quoted string to a [CodeBuffer] in
    * a form that is valid as JavaScript string literal content.
    * The string is assumed quoted by double quote characters.
    */
   @override
   jsAst.Expression visitString(StringConstantValue constant, [_]) {
-    StringBuffer sb = new StringBuffer();
-    writeJsonEscapedCharsOn(constant.primitiveValue.slowToString(), sb);
-    return new jsAst.LiteralString('"$sb"');
+    return js.escapedString(constant.primitiveValue.slowToString(),
+                            ascii: true);
   }
 
   @override
   jsAst.Expression visitList(ListConstantValue constant, [_]) {
     List<jsAst.Expression> elements = constant.entries
         .map(constantReferenceGenerator)
         .toList(growable: false);
     jsAst.ArrayInitializer array = new jsAst.ArrayInitializer(elements);
     jsAst.Expression value = makeConstantList(array);
     return maybeAddTypeArguments(constant.type, value);
@@ skipping 151 matching lines @@
           [value, argumentList]);
     }
     return value;
   }
 
   @override
   jsAst.Expression visitDeferred(DeferredConstantValue constant, [_]) {
     return constantReferenceGenerator(constant.referenced);
   }
 }