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/ios/mac.dart b/packages/flutter_tools/lib/src/ios/mac.dart
index 644f66a..ee60c27 100644
--- a/packages/flutter_tools/lib/src/ios/mac.dart
+++ b/packages/flutter_tools/lib/src/ios/mac.dart
@@ -95,7 +95,7 @@
   /// Starts `idevicesyslog` and returns the running process.
   Future<Process> startLogger() => runCommand(<String>['idevicesyslog']);
 
-  /// Captures a screenshot to the specified outputfile.
+  /// Captures a screenshot to the specified outputFile.
   Future<Null> takeScreenshot(File outputFile) {
     return runCheckedAsync(<String>['idevicescreenshot', outputFile.path]);
   }