Add File.lengthSync to use it for FileByteStore eviction.

pkg/analyzer/lib/file_system/file_system.dart

Index: pkg/analyzer/lib/file_system/file_system.dart
diff --git a/pkg/analyzer/lib/file_system/file_system.dart b/pkg/analyzer/lib/file_system/file_system.dart
index ac8c0db6652a0ee187ba92c9640b1f2294cbb1f3..139330bb61740fa24900c03c859c30520b3baf86 100644
--- a/pkg/analyzer/lib/file_system/file_system.dart
+++ b/pkg/analyzer/lib/file_system/file_system.dart
@@ -21,8 +21,14 @@ abstract class File implements Resource {
   Stream<WatchEvent> get changes;
 
   /**
+   * Synchronously get the length of the file.
+   * Throws a [FileSystemException] if the operation fails.
+   */
+  int get lengthSync;
+
+  /**
    * Return the last-modified stamp of the file.
-   * Throws [FileSystemException] if the file does not exist.
+   * Throws a [FileSystemException] if the file does not exist.
    */
   int get modificationStamp;