Allow collection of trace files when adbd is not running as root

Also fix a bug where the trace command may capture the wrong file
if multiple trace file paths are in the Android log buffer.

Previously we found a lower bound timestamp for the trace path log
by running the date command on the device and parsing the result on
the host.  This could yield an inaccurate result if the device and
host are using different time zones.

The command will now obtain the most recent timestamp in the device's
time format by running logcat.

packages/flutter_tools/lib/src/android/device_android.dart

@@ -447,47 +447,20 @@ class AndroidDevice extends Device {
     ]));
   }
 
-  static String _threeDigits(int n) {
-    if (n >= 100) return "$n";
-    if (n >= 10) return "0$n";
-    return "00$n";
+  // Return the most recent timestamp in the Android log.  The format can be
+  // passed to logcat's -T option.
+  String lastLogcatTimestamp() {
+    String output = runCheckedSync(adbCommandForDevice(['logcat', '-v', 'time', '-t', '1']));
+
+    RegExp timeRegExp = new RegExp(r'^\d{2}-\d{2} \d{2}:\d{2}:\d{2}\.\d{3}', multiLine: true);
+    Match timeMatch = timeRegExp.firstMatch(output);
+    return timeMatch[0];
   }
 
-  static String _twoDigits(int n) {
-    if (n >= 10) return "$n";
-    return "0$n";
-  }
-
-  static String _logcatDateFormat(DateTime dt) {
-    // Doing this manually, instead of using package:intl for simplicity.
-    // adb logcat -T wants "%m-%d %H:%M:%S.%3q"
-    String m = _twoDigits(dt.month);
-    String d = _twoDigits(dt.day);
-    String H = _twoDigits(dt.hour);
-    String M = _twoDigits(dt.minute);
-    String S = _twoDigits(dt.second);
-    String q = _threeDigits(dt.millisecond);
-    return "$m-$d $H:$M:$S.$q";
-  }
-
-  // TODO(eseidel): This is fragile, there must be a better way!
-  DateTime timeOnDevice() {
-    // Careful: Android's date command is super-lame, any arguments are taken as
-    // attempts to set the timezone and will screw your device.
-    String output = runCheckedSync(adbCommandForDevice(['shell', 'date'])).trim();
-    // format: Fri Dec 18 13:22:07 PST 2015
-    // intl doesn't handle timezones: https://github.com/dart-lang/intl/issues/93
-    // So we use the local date command to parse dates for us.
-    String seconds = runSync(['date', '--date', output, '+%s']);
-    // Although '%s' is supposed to be UTC, date appears to be ignoring the
-    // timezone in the passed string, so using isUTC: false here.
-    return new DateTime.fromMillisecondsSinceEpoch(int.parse(seconds) * 1000, isUtc: false);
-  }
-
-  String stopTracing(AndroidApk apk, { String outPath: null }) {
+  Future<String> stopTracing(AndroidApk apk, { String outPath: null }) async {
     // Workaround for logcat -c not always working:
     // http://stackoverflow.com/questions/25645012/logcat-on-android-l-not-clearing-after-unplugging-and-reconnecting
-    String beforeStop = _logcatDateFormat(timeOnDevice());
+    String beforeStop = lastLogcatTimestamp();
     runCheckedSync(adbCommandForDevice([
       'shell',
       'am',
@@ -512,10 +485,23 @@ class AndroidDevice extends Device {
 
     if (tracePath != null) {
       String localPath = (outPath != null) ? outPath : path.basename(tracePath);
-      runCheckedSync(adbCommandForDevice(['root']));
-      runSync(adbCommandForDevice(['shell', 'run-as', apk.id, 'chmod', '777', tracePath]));
-      runCheckedSync(adbCommandForDevice(['pull', tracePath, localPath]));
-      runSync(adbCommandForDevice(['shell', 'rm', tracePath]));
+
+      // Run cat via ADB to print the captured trace file.  (adb pull will be unable
+      // to access the file if it does not have root permissions)
+      IOSink catOutput = new File(localPath).openWrite();
+      List<String> catCommand = adbCommandForDevice(
+          <String>['shell', 'run-as', apk.id, 'cat', tracePath]
+      );
+      Process catProcess = await Process.start(catCommand[0],
+          catCommand.getRange(1, catCommand.length).toList());
+      catProcess.stdout.pipe(catOutput);
+      int exitCode = await catProcess.exitCode;
+      if (exitCode != 0)
+        throw 'Error code $exitCode returned when running ${catCommand.join(" ")}';
+
+      runSync(adbCommandForDevice(
+          <String>['shell', 'run-as', apk.id, 'rm', tracePath]
+      ));
       return localPath;
     }
     logging.warning('No trace file detected. '

packages/flutter_tools/lib/src/commands/trace.dart

@@ -43,17 +43,18 @@ class TraceCommand extends FlutterCommand {
       devices.android.startTracing(androidApp);
       await new Future.delayed(
           new Duration(seconds: int.parse(argResults['duration'])),
-          () => _stopTracing(devices.android, androidApp));
+          () => _stopTracing(devices.android, androidApp)
+      );
     } else if (argResults['stop']) {
-      _stopTracing(devices.android, androidApp);
+      await _stopTracing(devices.android, androidApp);
     } else {
       devices.android.startTracing(androidApp);
     }
     return 0;
   }
 
-  void _stopTracing(AndroidDevice android, AndroidApk androidApp) {
-    String tracePath = android.stopTracing(androidApp, outPath: argResults['out']);
+  Future _stopTracing(AndroidDevice android, AndroidApk androidApp) async {
+    String tracePath = await android.stopTracing(androidApp, outPath: argResults['out']);
     if (tracePath == null) {
       logging.warning('No trace file saved.');
     } else {