Add File.lengthSync to use it for FileByteStore eviction.

pkg/analyzer/lib/file_system/memory_file_system.dart

Index: pkg/analyzer/lib/file_system/memory_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/memory_file_system.dart b/pkg/analyzer/lib/file_system/memory_file_system.dart
index 44f8a062661923f70ce486d50f28fc451d42d679..42731be1b09940893addb31a4f68e197b71bcd2d 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -306,6 +306,11 @@ class _MemoryDummyLink extends _MemoryResource implements File {
   bool get exists => false;
 
   @override
+  int get lengthSync {
+    throw new FileSystemException(path, 'File could not be read');
+  }
+
+  @override
   int get modificationStamp {
     int stamp = _provider._pathToTimestamp[path];
     if (stamp == null) {
@@ -371,6 +376,11 @@ class _MemoryFile extends _MemoryResource implements File {
   bool get exists => _provider._pathToResource[path] is _MemoryFile;
 
   @override
+  int get lengthSync {
+    return readAsBytesSync().length;
+  }
+
+  @override
   int get modificationStamp {
     int stamp = _provider._pathToTimestamp[path];
     if (stamp == null) {