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

tests/compiler/dart2js/string_escapes2_test.dart

+// Copyright (c) 2015, 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.
+
+import 'dart:async';
+import 'package:async_helper/async_helper.dart';
+import 'package:expect/expect.dart';
+import 'compiler_helper.dart';
+
+// Test that the compiler doesn't escape too many characters.
+
+Future<String> compileExpression(String expression) {
+  var source = "foo() { return $expression; }";
+  return compile(source, entry: "foo");
+}
+
+Future runTest() {
+  return compileExpression(r"'Тест на Кирилица - great.\n"
+                           r'Next "line".\u2028'
+                           r'And another one.\u2029'
+                           r"and the last one'")
+      .then((String generated) {
+    Expect.isTrue(
+        generated.contains(r'"Тест на Кирилица - great.\n'
+                           r'Next \"line\".\u2028'
+                           r'And another one.\u2029'
+                           r'and the last one"') ||
+        generated.contains(r"'Тест на Кирилица - great.\n"
+                           r'Next "line".\u2028'
+                           r'And another one.\u2029'
+                           r"and the last one'"));
+  });
+}
+
+main() {
+  asyncTest(runTest);
+}