Add instrumentation data to the analysis server diagnostics page.

pkg/analyzer/lib/instrumentation/file_instrumentation.dart

Index: pkg/analyzer/lib/instrumentation/file_instrumentation.dart
diff --git a/pkg/analyzer/lib/instrumentation/file_instrumentation.dart b/pkg/analyzer/lib/instrumentation/file_instrumentation.dart
index d4088f87fd145cb493c5b0103c0462f3c53fdc1e..32133f6505c5cff5eae17db339d76f9b31b65cbf 100644
--- a/pkg/analyzer/lib/instrumentation/file_instrumentation.dart
+++ b/pkg/analyzer/lib/instrumentation/file_instrumentation.dart
@@ -13,10 +13,11 @@ import 'package:analyzer/instrumentation/instrumentation.dart';
  * An [InstrumentationServer] that writes to a file.
  */
 class FileInstrumentationServer implements InstrumentationServer {
+  final String filePath;
   IOSink _sink;
 
-  FileInstrumentationServer(String path) {
-    File file = new File(path);
+  FileInstrumentationServer(this.filePath) {
+    File file = new File(filePath);
     _sink = file.openWrite();
   }
 
@@ -24,6 +25,9 @@ class FileInstrumentationServer implements InstrumentationServer {
   String get sessionId => '';
 
   @override
+  String get describe => "file: $filePath";
+
+  @override
   void log(String message) {
     _sink.writeln(message);
   }