dart2js: Avoid escaping in strings if it's not necessary.

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;
 
 class ConstantEmitter {
   ConstantReferenceEmitter _referenceEmitter;
   ConstantLiteralEmitter _literalEmitter;
 
@@ skipping 269 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());
   }
 
   @override
   jsAst.Expression visitList(ListConstantValue constant, [_]) {
     List<jsAst.Expression> elements = _array(constant.entries);
     jsAst.ArrayInitializer array = new jsAst.ArrayInitializer(elements);
     jsAst.Expression value = makeConstantListTemplate.instantiate([array]);
     return maybeAddTypeArguments(constant.type, value);
   }
 
@@ skipping 141 matching lines @@
                             [value, argumentList]);
     }
     return value;
   }
 
   @override
   jsAst.Expression visitDeferred(DeferredConstantValue constant, [_]) {
     return constantEmitter.reference(constant.referenced);
   }
 }