Add support for getting the sessionId from instrumentation and sending it to the client

pkg/analyzer/test/instrumentation/instrumentation_test.dart

Index: pkg/analyzer/test/instrumentation/instrumentation_test.dart
diff --git a/pkg/analyzer/test/instrumentation/instrumentation_test.dart b/pkg/analyzer/test/instrumentation/instrumentation_test.dart
index 8386ee70f4f3a3c6463e2aa0e55277a8683f1927..e6cafc002b54c02e9eef3910fdec7a164b239bef 100644
--- a/pkg/analyzer/test/instrumentation/instrumentation_test.dart
+++ b/pkg/analyzer/test/instrumentation/instrumentation_test.dart
@@ -165,6 +165,9 @@ class TestInstrumentationServer implements InstrumentationServer {
   StringBuffer priorityChannel = new StringBuffer();
 
   @override
+  String get sessionId => '';
+
+  @override
   void log(String message) {
     normalChannel.writeln(message);
   }