GN: Make file/dir resolving return errors.

tools/gn/filesystem_utils_unittest.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/files/file_path.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
 #include "build/build_config.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "tools/gn/filesystem_utils.h"
@@ skipping 218 matching lines @@
   EXPECT_EQ("../../..", input);
 
   input = "../";
   NormalizePath(&input);
   EXPECT_EQ("../", input);
 
   // Backslash normalization.
   input = "foo\\..\\..\\bar";
   NormalizePath(&input);
   EXPECT_EQ("../bar", input);
+
+  // Trailing slashes should get preserved.
+  input = "//foo/bar/";
+  NormalizePath(&input);
+  EXPECT_EQ("//foo/bar/", input);
 }
 
 TEST(FilesystemUtils, RebasePath) {
   base::StringPiece source_root("/source/root");
 
   // Degenerate case.
   EXPECT_EQ(".", RebasePath("//", SourceDir("//"), source_root));
   EXPECT_EQ(".", RebasePath("//foo/bar/", SourceDir("//foo/bar/"),
                             source_root));
 
@@ skipping 334 matching lines @@
   EXPECT_EQ("gen/", GetToolchainGenDirAsOutputFile(&settings).value());
   EXPECT_EQ("//obj/",
             GetOutputDirForSourceDir(&settings, SourceDir("//")).value());
   EXPECT_EQ("obj/",
             GetOutputDirForSourceDirAsOutputFile(
                 &settings, SourceDir("//")).value());
   EXPECT_EQ("gen/",
             GetGenDirForSourceDirAsOutputFile(
                 &settings, SourceDir("//")).value());
 }