Change the behavior of open on files to not truncate by default

runtime/bin/file.h

 // Copyright (c) 2011, 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.
 
 #ifndef BIN_FILE_H_
 #define BIN_FILE_H_
 
 #if defined(_WIN32)
 typedef signed __int64 int64_t;
 typedef unsigned __int8 uint8_t;
 #else
 #include <stdint.h>
 #endif
 
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
 #include <sys/types.h>
 
 // Forward declaration.
 class FileHandle;
 
 class File {
  public:
+  enum FileOpenMode {
+    kRead = 0,
+    kWrite = 1,
+    kTruncate = 1 << 2,
+    kWriteTruncate = kWrite | kTruncate
+  };
+
   ~File();
 
   // Read/Write attempt to transfer num_bytes to/from buffer. It returns
   // the number of bytes read/written.
   int64_t Read(void* buffer, int64_t num_bytes);
   int64_t Write(const void* buffer, int64_t num_bytes);
 
   // ReadFully and WriteFully do attempt to transfer num_bytes to/from
   // the buffer. In the event of short accesses they will loop internally until
   // the whole buffer has been transferred or an error occurs. If an error
@@ skipping 16 matching lines @@
   bool SetPosition(int64_t position);
 
   // Truncate (or extend) the file to the given length in bytes.
   bool Truncate(int64_t length);
 
   // Flush contents of file.
   void Flush();
 
   const char* name() const { return name_; }
 
-  static File* Open(const char* name, bool writable);
+  // Open the file with the given name. The file is always opened for
+  // reading. If mode contains kWrite the file is opened for both
+  // reading and writing. If mode contains kWrite and the file does
+  // not exist the file is created. The file is truncated to length 0 if
+  // mode contains kTruncate.
+  static File* Open(const char* name, FileOpenMode mode);
+
   static bool Exists(const char* name);
   static bool Create(const char* name);
   static bool Delete(const char* name);
   static bool IsAbsolutePath(const char* pathname);
   static char* GetCanonicalPath(const char* name);
   static const char* PathSeparator();
   static const char* StringEscapedPathSeparator();
 
  private:
   File(const char* name, FileHandle* handle) : name_(name), handle_(handle) { }
   void Close();
   bool IsClosed();
 
   static const int kClosedFd = -1;
 
   const char* name_;
   // FileHandle is an OS specific class which stores data about the file.
   FileHandle* handle_;  // OS specific handle for the file.
 
   // DISALLOW_COPY_AND_ASSIGN(File).
   File(const File&);
   void operator=(const File&);
 };
 
 #endif  // BIN_FILE_H_