Implement `gn analyze`.

tools/gn/filesystem_utils.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 "tools/gn/filesystem_utils.h"
 
 #include <algorithm>
 
 #include "base/files/file_util.h"
 #include "base/logging.h"
@@ skipping 318 matching lines @@
   }
 
   // Double forward slash at the beginning means source-relative (we don't
   // allow backslashes for denoting this).
   if (path.size() > 1 && path[1] == '/')
     return false;
 
   return true;
 }
 
+bool IsPathSourceAbsolute(const base::StringPiece& path) {
+  return (path.size() >= 2 && path[0] == '/' && path[1] == '/');
+}
+
 bool MakeAbsolutePathRelativeIfPossible(const base::StringPiece& source_root,
                                         const base::StringPiece& path,
                                         std::string* dest) {
   DCHECK(IsPathAbsolute(source_root));
   DCHECK(IsPathAbsolute(path));
 
   dest->clear();
 
   if (source_root.size() > path.size())
     return false;  // The source root is longer: the path can never be inside.
@@ skipping 398 matching lines @@
 
   return file_data == data;
 }
 
 bool WriteFileIfChanged(const base::FilePath& file_path,
                         const std::string& data,
                         Err* err) {
   if (ContentsEqual(file_path, data))
     return true;
 
+  return WriteFile(file_path, data, err);
+}
+
+bool WriteFile(const base::FilePath& file_path, const std::string& data,
+               Err* err) {
   // Create the directory if necessary.
   if (!base::CreateDirectory(file_path.DirName())) {
     if (err) {
       *err =
           Err(Location(), "Unable to create directory.",
               "I was using \"" + FilePathToUTF8(file_path.DirName()) + "\".");
     }
     return false;
   }
 
@@ skipping 130 matching lines @@
       BuildDirContext(target), target->label().dir(), type);
 }
 
 SourceDir GetScopeCurrentBuildDirAsSourceDir(const Scope* scope,
                                              BuildDirType type) {
   if (type == BuildDirType::TOOLCHAIN_ROOT)
     return GetBuildDirAsSourceDir(BuildDirContext(scope), type);
   return GetSubBuildDirAsSourceDir(
       BuildDirContext(scope), scope->GetSourceDir(), type);
 }