Add File.lengthSync to use it for FileByteStore eviction.

pkg/analyzer/test/file_system/physical_resource_provider_test.dart

Index: pkg/analyzer/test/file_system/physical_resource_provider_test.dart
diff --git a/pkg/analyzer/test/file_system/physical_resource_provider_test.dart b/pkg/analyzer/test/file_system/physical_resource_provider_test.dart
index 39ac5dcfa4d41091f729dff256fc42aa28e82ac6..62b4bbafd0de1bae1d3ed5a3d13d883c8124e6a5 100644
--- a/pkg/analyzer/test/file_system/physical_resource_provider_test.dart
+++ b/pkg/analyzer/test/file_system/physical_resource_provider_test.dart
@@ -97,6 +97,19 @@ class FileTest extends _BaseTest {
     expect(file.isOrContains('foo'), isFalse);
   }
 
+  void test_lengthSync_doesNotExist() {
+    File file = PhysicalResourceProvider.INSTANCE.getResource(path);
+    expect(() {
+      file.lengthSync;
+    }, throwsA(_isFileSystemException));
+  }
+
+  void test_lengthSync_exists() {
+    List<int> bytes = <int>[1, 2, 3, 4, 5];
+    new io.File(path).writeAsBytesSync(bytes);
+    expect(file.lengthSync, bytes.length);
+  }
+
   void test_modificationStamp_doesNotExist() {
     File file = PhysicalResourceProvider.INSTANCE.getResource(path);
     expect(() {