Implement `gn analyze`.

tools/gn/filesystem_utils.cc

Index: tools/gn/filesystem_utils.cc
diff --git a/tools/gn/filesystem_utils.cc b/tools/gn/filesystem_utils.cc
index 6152ae04656a5b40a6adc58f1391da28bcb74849..44fa0eb505f523cbd6ea039f737d874326c1bd28 100644
--- a/tools/gn/filesystem_utils.cc
+++ b/tools/gn/filesystem_utils.cc
@@ -336,6 +336,10 @@ bool IsPathAbsolute(const base::StringPiece& path) {
   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) {
@@ -754,6 +758,11 @@ bool WriteFileIfChanged(const base::FilePath& file_path,
   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) {