send channel name as a custom dimension (#10814)

* send channel name as a custom dimension

* tweaks from review

* remove unused import

packages/flutter_tools/lib/src/usage.dart

@@ -26,8 +26,9 @@ class Usage {
     _analytics = new AnalyticsIO(_kFlutterUA, settingsName, version);
 
     // Report a more detailed OS version string than package:usage does by
-    // default as custom dimension 1 (configured in our analytics account).
+    // default. Also, send the branch name as the "channel".
     _analytics.setSessionValue('dimension1', os.name);
+    _analytics.setSessionValue('dimension2', FlutterVersion.getBranchName(whitelistBranchName: true));
 
     bool runningOnCI = false;
 

packages/flutter_tools/lib/src/version.dart

@@ -149,6 +149,16 @@ class FlutterVersion {
     String commit = _shortGitRevision(_runSync(<String>['git', 'rev-parse', 'HEAD']));
     commit = commit.isEmpty ? 'unknown' : commit;
 
+    final String branch = getBranchName(whitelistBranchName: whitelistBranchName);
+
+    return '$branch/$commit';
+  }
+
+  /// Return the branch name.
+  ///
+  /// If whitelistBranchName is true and the branch is unknown,
+  /// the branch name will be returned as 'dev'.
+  static String getBranchName({ bool whitelistBranchName: false }) {
     String branch = _runSync(<String>['git', 'rev-parse', '--abbrev-ref', 'HEAD']);
     branch = branch == 'HEAD' ? 'master' : branch;
 
@@ -158,7 +168,7 @@ class FlutterVersion {
         branch = 'dev';
     }
 
-    return '$branch/$commit';
+    return branch;
   }
 
   /// The amount of time we wait before pinging the server to check for the