Make it possible to specify the saved path for trace --stop

@chinmaygarde

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

@@ -20,6 +20,7 @@ class TraceCommand extends FlutterCommand {
   TraceCommand() {
     argParser.addFlag('start', negatable: false, help: 'Start tracing.');
     argParser.addFlag('stop', negatable: false, help: 'Stop tracing.');
+    argParser.addOption('out', help: 'Specify the path of the saved trace file.');
     argParser.addOption('duration',
         defaultsTo: '10', abbr: 'd', help: 'Duration in seconds to trace.');
   }
@@ -52,7 +53,7 @@ class TraceCommand extends FlutterCommand {
   }
 
   void _stopTracing(AndroidDevice android, AndroidApk androidApp) {
-    String tracePath = android.stopTracing(androidApp);
+    String tracePath = android.stopTracing(androidApp, outPath: argResults['out']);
     if (tracePath == null) {
       logging.warning('No trace file saved.');
     } else {

packages/flutter_tools/lib/src/device.dart

@@ -853,7 +853,7 @@ class AndroidDevice extends Device {
     ]));
   }
 
-  String stopTracing(AndroidApk apk) {
+  String stopTracing(AndroidApk apk, { String outPath: null }) {
     clearLogs();
     runCheckedSync(adbCommandForDevice([
       'shell',
@@ -878,11 +878,12 @@ 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]));
+      runCheckedSync(adbCommandForDevice(['pull', tracePath, localPath]));
       runSync(adbCommandForDevice(['shell', 'rm', tracePath]));
-      return path.basename(tracePath);
+      return localPath;
     }
     logging.warning('No trace file detected. '
         'Did you remember to start the trace before stopping it?');