enable analysis server cmdline instrumentation log file option

pkg/analysis_server/lib/src/server/driver.dart

Index: pkg/analysis_server/lib/src/server/driver.dart
diff --git a/pkg/analysis_server/lib/src/server/driver.dart b/pkg/analysis_server/lib/src/server/driver.dart
index 1862b9ff4fbc677d5e08108d1797bbe27760917c..d71ed5bb062bc29de4b74ad389a0922ccc05b137 100644
--- a/pkg/analysis_server/lib/src/server/driver.dart
+++ b/pkg/analysis_server/lib/src/server/driver.dart
@@ -381,12 +381,14 @@ class Driver implements ServerStarter {
     //
     // Initialize the instrumentation service.
     //
+    String logFilePath = results[INSTRUMENTATION_LOG_FILE];
     if (instrumentationServer != null) {
-      String filePath = results[INSTRUMENTATION_LOG_FILE];
-      if (filePath != null) {
+      if (logFilePath != null) {

[lukechurch, 2015/10/23 19:28:35]

The pattern here is a little hard to read. Could we clarify a bit by extracting
the common if.

if (logFilePath != null) {
  fileBasedServer = new FileInstrumentationServer(logFilePath);
  
  if (instrumentationServer != null) {
    ...... multicast CTOR
  } else { 
     instrumentationServer = fileBasedServer;
}

         instrumentationServer = new MulticastInstrumentationServer(
-            [instrumentationServer, new FileInstrumentationServer(filePath)]);
+            [instrumentationServer, new FileInstrumentationServer(logFilePath)]);
       }
+    } else if (logFilePath != null) {
+      instrumentationServer = new FileInstrumentationServer(logFilePath);
     }
     InstrumentationService service =
         new InstrumentationService(instrumentationServer);