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

pkg/analyzer/lib/instrumentation/instrumentation.dart

Index: pkg/analyzer/lib/instrumentation/instrumentation.dart
diff --git a/pkg/analyzer/lib/instrumentation/instrumentation.dart b/pkg/analyzer/lib/instrumentation/instrumentation.dart
index eaf95ba56e87dcb58533c1c65fffc63c39c11de9..abc2799315e8927bd90df7a6999f164dedddb0e3 100644
--- a/pkg/analyzer/lib/instrumentation/instrumentation.dart
+++ b/pkg/analyzer/lib/instrumentation/instrumentation.dart
@@ -23,6 +23,11 @@ class AnalysisPerformanceKind {
  */
 abstract class InstrumentationServer {
   /**
+   * Return the identifier used to identify the current session.
+   */
+  String get sessionId;
+
+  /**
    * Pass the given [message] to the instrumentation server so that it will be
    * logged with other messages.
    *
@@ -96,6 +101,11 @@ class InstrumentationService {
   bool get isActive => _instrumentationServer != null;
 
   /**
+   * Return the identifier used to identify the current session.
+   */
+  String get sessionId => _instrumentationServer?.sessionId ?? '';
+
+  /**
    * The current time, expressed as a decimal encoded number of milliseconds.
    */
   String get _timestamp => new DateTime.now().millisecondsSinceEpoch.toString();
@@ -357,6 +367,9 @@ class MulticastInstrumentationServer implements InstrumentationServer {
   MulticastInstrumentationServer(this._servers);
 
   @override
+  String get sessionId => _servers[0].sessionId;
+
+  @override
   void log(String message) {
     for (InstrumentationServer server in _servers) {
       server.log(message);