Add FileSystemEntity.identical, to test if two file system objects are the same.

runtime/bin/file.h

Index: runtime/bin/file.h
diff --git a/runtime/bin/file.h b/runtime/bin/file.h
index 719239caecc833e907b13eaf035fd1b31a0f2a1c..a4c6d4bb5f59d68846d0c223264cc68c16bebd7c 100644
--- a/runtime/bin/file.h
+++ b/runtime/bin/file.h
@@ -43,6 +43,12 @@ class File {
     kDoesNotExist = 3
   };
 
+  enum Identical {
+    kIdentical = 0,
+    kDifferent = 1,
+    kError = 2
+  };
+
   enum StdioHandleType {
     kTerminal = 0,
     kPipe = 1,
@@ -134,6 +140,7 @@ class File {
   static const char* PathSeparator();
   static const char* StringEscapedPathSeparator();
   static Type GetType(const char* path, bool follow_links);
+  static Identical AreIdentical(const char* file_1, const char* file_2);
   static StdioHandleType GetStdioHandleType(int fd);
 
   static FileOpenMode DartModeToFileMode(DartFileOpenMode mode);