Add File.lengthSync to use it for FileByteStore eviction.

pkg/analyzer/lib/file_system/physical_file_system.dart

Index: pkg/analyzer/lib/file_system/physical_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/physical_file_system.dart b/pkg/analyzer/lib/file_system/physical_file_system.dart
index a9cf94b99734395d41127df071264e0c23f075f3..411d0d1a6d9961a54502e8588993cb62c1ec2a8f 100644
--- a/pkg/analyzer/lib/file_system/physical_file_system.dart
+++ b/pkg/analyzer/lib/file_system/physical_file_system.dart
@@ -127,6 +127,15 @@ class _PhysicalFile extends _PhysicalResource implements File {
   Stream<WatchEvent> get changes => new FileWatcher(_entry.path).events;
 
   @override
+  int get lengthSync {
+    try {
+      return _file.lengthSync();
+    } on io.FileSystemException catch (exception) {
+      throw new FileSystemException(exception.path, exception.message);
+    }
+  }
+
+  @override
   int get modificationStamp {
     try {
       return _file.lastModifiedSync().millisecondsSinceEpoch;