performance measurement improvements and README update

pkg/analysis_server/benchmark/integration/README.md

 # Running Benchmarks
 
 There are two entry points for running benchmarks:
 * **main.dart** - a general Dart application for running performance benchmarks
 * **local_runner.dart** - an example Dart application
 which sets up the local environment
 and then calls main.dart to run performance benchmarks
 
 ## local_runner.dart
 
 This Dart application is one example for running performance benchmarks.
 When run, this application 1) extracts a branch from a git repository
 into a temporary directory, and 2) creates a symlink to the out or xcodebuild
 directory for proper package-root package resolution.
+Once setup is complete, this applications calls main.dart
+
 The required command line arguments are
 * **gitDir** = a path to the git repository containing the initial target source
 * **branch** = the branch containing the initial target source
 * **inputFile** = the instrumentation or log file
 
-Once setup is complete, this applications calls main.dart
+Additional arguments are passed directly to main.dart.
+For example, you may want to specify --newTaskModel to measure performance
+with the new task model versus the old task model,
+or if the log was recorded on one machine and is played back on another,
+then you might need to specify -m<oldSrcPath>,<newSrcPath>
+to map the source paths for playback.
+When specifying additional arguments, any occurrences of @tmpSrcDir@
+will be replaced with the absolute path of the temporary directory
+into which the source was extracted.
 
 ## main.dart
 
 This Dart application reads an instrumentation or local log file produced by
 analysis server, "replays" that interaction with the analysis server,
 compares the notifications and responses with what was recorded in the log,
 and produces a report. It assumes that the environment for playback has
 already been setup.
 The required command line arguments are
 *  **-i, --input             <filePath>**
 The input file specifying how this client should interact with the server.
 If the input file name is "stdin", then the instructions are read from stdin.
 *  **-m, --map               <oldSrcPath>,<newSrcPath>**
 This option defines a mapping from the original source directory <oldSrcPath>
 when the instrumentation or log file was generated
 to the target source directory <newSrcPath> used during performance testing.
 Multiple mappings can be specified.
 WARNING: The contents of the target directory will be modified
 *  **-t, --tmpSrcDir         <dirPath>**
 The temporary directory containing source used during performance measurement.
 WARNING: The contents of the target directory will be modified
 *  **--newTaskModel**       enable the use of the new task model
 *  **-d, --diagnosticPort** localhost port on which server
                             will provide diagnostic web pages
 *  **-v, --verbose**        Verbose logging
 *  **--vv**                 Extra verbose logging
 *  **-h, --help**           Print this help information
+
+For each request recorded in the input file,
+the application sends a corresponding request to the analysis server
+and waits up to 60 seconds for a response to that request.
+If a response in not received in that time, then the application exits.
+Any responses that are received are compared with the recorded response.
+
+For each analysis-complete notification recorded in the input file,
+the application waits for the corresponding analysis-complete notification
+from the running analysis server.
+While it is waiting for an analysis-complete notification,
+the application monitors the stream of notifications.
+If there is a period of more than 60 seconds during which no communication
+is received from the server, the application assumes that the server is hung
+and exits.