Improve error message when missing the package-root

packages/flutter_tools/lib/src/artifacts.dart

@@ -23,8 +23,11 @@ class ArtifactStore {
   static String _engineRevision;
 
   static String get engineRevision {
-    if (_engineRevision == null)
-      _engineRevision = new File(path.join(packageRoot, 'sky_engine', 'REVISION')).readAsStringSync();
+    if (_engineRevision == null) {
+      File revisionFile = new File(path.join(packageRoot, 'sky_engine', 'REVISION'));
+      if (revisionFile.existsSync())
+        _engineRevision = revisionFile.readAsStringSync();
+    }
     return _engineRevision;
   }
 

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

@@ -88,6 +88,18 @@ class FlutterCommandRunner extends CommandRunner {
       Logger.root.level = Level.FINE;
 
     ArtifactStore.packageRoot = globalResults['package-root'];
+    if (!FileSystemEntity.isDirectorySync(ArtifactStore.packageRoot)) {
+      String message = '${ArtifactStore.packageRoot} is not a valid directory.';
+      if (ArtifactStore.packageRoot == 'packages') {
+        if (FileSystemEntity.isFileSync('pubspec.yaml'))
+          message += '\nDid you run `pub get` in this directory?';
+        else
+          message += '\nDid you run this command from the same directory as your pubspec.yaml file?';
+      }
+      _logging.severe(message);
+      return 2;
+    }
+
     buildConfigurations = _createBuildConfigurations(globalResults);
 
     return super.runCommand(globalResults);