run_mojo: Add --mojo-release and --mojo-debug flags.

The flags control which build of mojo to run (Debug or Release).
Default is Release.

packages/flutter_tools/lib/src/run_mojo.dart

@@ -22,6 +22,9 @@ class RunMojoCommand extends Command {
   final description = 'Run a Flutter app in mojo.';
   RunMojoCommand() {
     argParser.addFlag('android', negatable: false, help: 'Run on an Android device');
+    argParser.addFlag('mojo-debug', negatable: false, help: 'Use Debug build of mojo');
+    argParser.addFlag('mojo-release', negatable: false, help: 'Use Release build of mojo (default)');
+
     argParser.addOption('app', defaultsTo: 'app.flx');
     argParser.addOption('mojo-path', help: 'Path to directory containing mojo_shell and services');
     argParser.addOption('package-root', defaultsTo: 'packages');
@@ -40,9 +43,10 @@ class RunMojoCommand extends Command {
     String command = await _makePathAbsolute(path.join(results['mojo-path'], 'mojo', 'devtools', 'common', 'mojo_run'));
     String appName = path.basename(appPath);
     String appDir = path.dirname(appPath);
+    String buildFlag = argResults['mojo-debug'] ? '--debug' : '--release';
     List<String> args = [
       '--android',
-      '--release',
+      buildFlag,
       'http://app/$appName',
       '--map-origin=http://app/=$appDir',
       '--map-origin=http://sky_viewer/=$skyViewerUrl',
@@ -60,7 +64,8 @@ class RunMojoCommand extends Command {
 
   Future<int> _runLinux(ArgResults results, String appPath, ArtifactStore artifacts) async {
     String viewerPath = await _makePathAbsolute(await artifacts.getPath(Artifact.SkyViewerMojo));
-    String mojoShellPath = await _makePathAbsolute(path.join(results['mojo-path'], 'out', 'Release', 'mojo_shell'));
+    String mojoBuildType = argResults['mojo-debug'] ? 'Debug' : 'Release';
+    String mojoShellPath = await _makePathAbsolute(path.join(results['mojo-path'], 'out', mojoBuildType, 'mojo_shell'));
     List<String> args = [
       'file://${appPath}',
       '--url-mappings=mojo:sky_viewer=file://${viewerPath}'
@@ -75,6 +80,10 @@ class RunMojoCommand extends Command {
       _logging.severe('Must specify --mojo-path to mojo_run');
       return 1;
     }
+    if (argResults['mojo-debug'] && argResults['mojo-release']) {
+      _logging.severe('Cannot specify both --mojo-debug and --mojo-release');
+      return 1;
+    }
     String packageRoot = argResults['package-root'];
     ArtifactStore artifacts = new ArtifactStore(packageRoot);
     String appPath = await _makePathAbsolute(argResults['app']);