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

runtime/bin/file.dart

Index: runtime/bin/file.dart
diff --git a/runtime/bin/file.dart b/runtime/bin/file.dart
index 3ad0ec7186899c101a08adc492e959edb1135415..4fd8bbbfcd26655c4af433a1a671a99b4c4bcdbb 100644
--- a/runtime/bin/file.dart
+++ b/runtime/bin/file.dart
@@ -2,6 +2,18 @@
 // 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.
 
+
+/**
+ * FileMode describes the modes in which a file can be opened.
+ */
+class FileMode {
+  static final READ = const FileMode(0);
+  static final WRITE = const FileMode(1);
+  static final APPEND = const FileMode(2);
+  const FileMode(int this.mode);
+  final int mode;
+}
+
 interface File default _File {
   /**
    * Create a File object.
@@ -49,19 +61,48 @@ interface File default _File {
 
   /**
    * Open the file for random access operations. When the file is
-   * opened the openHandler is called with the resulting
+   * opened the [openHandler] is called with the resulting
    * RandomAccessFile. RandomAccessFiles must be closed using the
-   * close method. By default writable is false.
-   */
-  void open([bool writable]);
+   * [close] method. If the file cannot be opened the [errorHandler]
+   * is called.
+   *
+   * Files can be opened in three modes:
+   *
+   * FileMode.READ: open the file for reading. If the file does not
+   * exist the [errorHandler] is called.
+   *
+   * FileMode.WRITE: open the file for both reading and writing and
+   * truncate the file to length zero. If the file does not exist the
+   * file is created.
+   *
+   * FileMode.APPEND: same as FileMode.WRITE except that the file is
+   * not truncated.
+   *
+   * By default mode is FileMode.READ.
+   */
+  void open([FileMode mode]);
 
   /**
    * Synchronously open the file for random access operations. The
    * result is a RandomAccessFile on which random access operations
    * can be performed. Opened RandomAccessFiles must be closed using
-   * the close method. By default writable is false.
-   */
-  RandomAccessFile openSync([bool writable]);
+   * the [close] method.
+   *
+   * Files can be opened in three modes:
+   *
+   * FileMode.READ: open the file for reading. If the file does not
+   * exist the [errorHandler] is called.
+   *
+   * FileMode.WRITE: open the file for both reading and writing and
+   * truncate the file to length zero. If the file does not exist the
+   * file is created.
+   *
+   * FileMode.APPEND: same as FileMode.WRITE except that the file is
+   * not truncated.
+   *
+   * By default mode is FileMode.READ.
+   */
+  RandomAccessFile openSync([FileMode mode]);
 
   /**
    * Get the canonical full path corresponding to the file name. The