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

runtime/bin/file_impl.dart

 // 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.
 
 class _FileInputStream extends _BaseDataInputStream implements InputStream {
   _FileInputStream(File file) {
     _file = file.openSync();
     _length = _file.lengthSync();
     _streamMarkedClosed = true;
     _checkScheduleCallbacks();
@@ skipping 32 matching lines @@
   }
 
   RandomAccessFile _file;
   int _length;
   bool _closed = false;
 }
 
 
 class _FileOutputStream implements OutputStream {
   _FileOutputStream(File file) {
-    _file = file.openSync(true);
+    _file = file.openSync(FileMode.WRITE);
   }
 
   bool write(List<int> buffer, [bool copyBuffer = false]) {
     return _write(buffer, 0, buffer.length);
   }
 
   bool writeFrom(List<int> buffer, [int offset = 0, int len]) {
     return _write(
         buffer, offset, (len == null) ? buffer.length - offset : len);
   }
@@ skipping 45 matching lines @@
 
   void execute(ReceivePort port) {
     _replyPort.send(_FileUtils.exists(_name), port.toSendPort());
   }
 
   String _name;
 }
 
 
 class _OpenOperation extends _FileOperation {
-  _OpenOperation(String this._name, bool this._writable);
+  _OpenOperation(String this._name, int this._mode);
 
   void execute(ReceivePort port) {
-    _replyPort.send(_FileUtils.checkedOpen(_name, _writable),
+    _replyPort.send(_FileUtils.checkedOpen(_name, _mode),
                     port.toSendPort());
   }
 
   String _name;
-  bool _writable;
+  int _mode;
 }
 
 
 class _CloseOperation extends _FileOperation {
   _CloseOperation(int this._id);
 
   void execute(ReceivePort port) {
     _replyPort.send(_FileUtils.close(_id), port.toSendPort());
   }
 
@@ skipping 266 matching lines @@
   }
 
   Queue<_FileOperation> _queue;
   _FileOperationIsolate _isolate;
 }
 
 
 // Helper class containing static file helper methods.
 class _FileUtils {
   static bool exists(String name) native "File_Exists";
-  static int open(String name, bool writable) native "File_Open";
+  static int open(String name, int mode) native "File_Open";
   static bool create(String name) native "File_Create";
   static bool delete(String name) native "File_Delete";
   static String fullPath(String name) native "File_FullPath";
   static int close(int id) native "File_Close";
   static int readByte(int id) native "File_ReadByte";
   static int readList(int id, List<int> buffer, int offset, int bytes)
       native "File_ReadList";
   static int writeByte(int id, int value) native "File_WriteByte";
   static int writeList(int id, List<int> buffer, int offset, int bytes)
       native "File_WriteList";
   static int writeString(int id, String string) native "File_WriteString";
   static int position(int id) native "File_Position";
   static bool setPosition(int id, int position) native "File_SetPosition";
   static bool truncate(int id, int length) native "File_Truncate";
   static int length(int id) native "File_Length";
   static int flush(int id) native "File_Flush";
 
-  static int checkedOpen(String name, bool writable) {
-    if (name is !String || writable is !bool) return 0;
-    return open(name, writable);
+  static int checkedOpen(String name, int mode) {
+    if (name is !String || mode is !int) return 0;
+    return open(name, mode);
   }
 
   static bool checkedCreate(String name) {
     if (name is !String) return false;
     return create(name);
   }
 
   static bool checkedDelete(String name) {
     if (name is !String) return false;
     return delete(name);
@@ skipping 93 matching lines @@
     if (_asyncUsed) {
       throw new FileIOException(
           "Mixed use of synchronous and asynchronous API");
     }
     bool deleted = _FileUtils.checkedDelete(_name);
     if (!deleted) {
       throw new FileIOException("Cannot delete file: $_name");
     }
   }
 
-  void open([bool writable = false]) {
+  void open([FileMode mode = FileMode.READ]) {
     _asyncUsed = true;
+    if (mode != FileMode.READ &&
+        mode != FileMode.WRITE &&
+        mode != FileMode.APPEND) {
+      if (_errorHandler != null) {
+        _errorHandler("Unknown file mode. Use FileMode.READ, FileMode.WRITE " +
+                      "or FileMode.APPEND.");
+        return;
+      }
+    }
     // If no open handler is present, close the file immediately to
     // avoid leaking an open file descriptor.
     var handler = _openHandler;
     if (handler === null) {
       handler = (file) => file.close();
     }
     var handleOpenResult = (id, ignored) {
       if (id != 0) {
         var randomAccessFile = new _RandomAccessFile(id, _name);
         handler(randomAccessFile);
       } else if (_errorHandler != null) {
         _errorHandler("Cannot open file: $_name");
       }
     };
-    var operation = new _OpenOperation(_name, writable);
+    var operation = new _OpenOperation(_name, mode.mode);
     _scheduler.enqueue(operation, handleOpenResult);
   }
 
-  void openSync([bool writable = false]) {
+  void openSync([FileMode mode = FileMode.READ]) {
     if (_asyncUsed) {
       throw new FileIOException(
           "Mixed use of synchronous and asynchronous API");
     }
-    var id = _FileUtils.checkedOpen(_name, writable);
+    if (mode != FileMode.READ &&
+        mode != FileMode.WRITE &&
+        mode != FileMode.APPEND) {
+      throw new FileIOException("Unknown file mode. Use FileMode.READ, " +
+                                "FileMode.WRITE or FileMode.APPEND.");
+    }
+    var id = _FileUtils.checkedOpen(_name, mode.mode);
     if (id == 0) {
       throw new FileIOException("Cannot open file: $_name");
     }
     return new _RandomAccessFile(id, _name);
   }
 
   void fullPath() {
     _asyncUsed = true;
     var handler = _fullPathHandler;
     if (handler == null) handler = (path) => null;
@@ skipping 464 matching lines @@
   var _readByteHandler;
   var _readListHandler;
   var _noPendingWriteHandler;
   var _positionHandler;
   var _setPositionHandler;
   var _truncateHandler;
   var _lengthHandler;
   var _flushHandler;
   var _errorHandler;
 }