Fix spelling errors in all the dartdocs. (#13061)

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.
diff --git a/packages/flutter_tools/lib/src/device.dart b/packages/flutter_tools/lib/src/device.dart
index dc85b43..e79c73f 100644
--- a/packages/flutter_tools/lib/src/device.dart
+++ b/packages/flutter_tools/lib/src/device.dart
@@ -389,7 +389,7 @@
   @override
   String toString() => name;
 
-  /// Process ID of the app on the deivce.
+  /// Process ID of the app on the device.
   int appPid;
 }