Fix handling of CRLF line endings in Script::GetSnippet.

tests/lib/mirrors/method_mirror_source_line_ending_crlf.dart

 // Copyright (c) 2014, 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.
 
-class A {
-  static var a;
-  var b;
+// This file is marked executable so source control will preserve the type of
+// line ending.
+
+library line_endings.crlf;
+
+oneLineCRLF(x) => x;
+multiLineCRLF(y) {
+  return y + 1;
 }
-
-class B extends A {
-
+c
+(){
 }
-
-class C extends B {
-  var a; /// 01: static type warning
-  static var b; /// 02: compile-time error
-}
-
-void main() {
-  new C();
-}