Updated README to notify users of Dart SDK version requirements.

lib/sync_http.dart

Index: lib/sync_http.dart
diff --git a/lib/sync_http.dart b/lib/sync_http.dart
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+++ b/lib/sync_http.dart
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+// Copyright (c) 2017, 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.
+
+library sync.http;
+
+import 'dart:convert';
+import 'dart:io'
+    show
+        BytesBuilder,
+        ContentType,
+        HttpException,
+        HttpHeaders,
+        RawSynchronousSocket;
+
+part 'src/sync_http.dart';
+part 'src/line_decoder.dart';