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

pkg/compiler/lib/src/js_backend/constant_emitter.dart

Index: pkg/compiler/lib/src/js_backend/constant_emitter.dart
diff --git a/pkg/compiler/lib/src/js_backend/constant_emitter.dart b/pkg/compiler/lib/src/js_backend/constant_emitter.dart
index 5b87b15b7854b9530c326d914669e032382792f6..0550d8d1002f1e624c5b93a764b02d629d0f3771 100644
--- a/pkg/compiler/lib/src/js_backend/constant_emitter.dart
+++ b/pkg/compiler/lib/src/js_backend/constant_emitter.dart
@@ -153,9 +153,8 @@ class ConstantEmitter
    */
   @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