Fix doc comment issues reported by pub.dev (#882)

Author: kevmoo

pkgs/args/README.md

@@ -446,6 +446,7 @@ The resulting string looks something like this:
 [ArgParser]: https://pub.dev/documentation/args/latest/args/ArgParser/ArgParser.html
 [ArgParserException]: https://pub.dev/documentation/args/latest/args/ArgParserException-class.html
 [ArgResults]: https://pub.dev/documentation/args/latest/args/ArgResults-class.html
+[ArgumentError]: https://api.dart.dev/dart-core/ArgumentError-class.html
 [CommandRunner]: https://pub.dev/documentation/args/latest/command_runner/CommandRunner-class.html
 [Command]: https://pub.dev/documentation/args/latest/command_runner/Command-class.html
 [UsageException]: https://pub.dev/documentation/args/latest/command_runner/UsageException-class.html

pkgs/async/lib/src/stream_group.dart

@@ -124,9 +124,9 @@ class StreamGroup<T> implements Sink<Stream<T>> {
 
   /// Adds [stream] as a member of this group.
   ///
-  /// Any events from [stream] will be emitted through [this.stream]. If this
-  /// group has a listener, [stream] will be listened to immediately; otherwise
-  /// it will only be listened to once this group gets a listener.
+  /// Any events from [stream] will be emitted through [StreamGroup.stream]. If
+  /// this group has a listener, [stream] will be listened to immediately;
+  /// otherwise it will only be listened to once this group gets a listener.
   ///
   /// If this is a single-subscription group and its subscription has been
   /// canceled, [stream] will be canceled as soon as its added. If this returns

pkgs/fixnum/lib/src/int32.dart

@@ -8,7 +8,7 @@ import 'int64.dart';
 import 'intx.dart';
 import 'utilities.dart' as u;
 
-/// An immutable 32-bit signed integer, in the range [-2^31, 2^31 - 1].
+/// An immutable 32-bit signed integer, in the range `[-2^31, 2^31 - 1]`.
 /// Arithmetic operations may overflow in order to maintain this range.
 class Int32 implements IntX {
   /// The maximum positive value attainable by an [Int32], namely

pkgs/fixnum/lib/src/int64.dart

@@ -13,7 +13,7 @@ import 'int32.dart';
 import 'intx.dart';
 import 'utilities.dart' as u;
 
-/// An immutable 64-bit signed integer, in the range [-2^63, 2^63 - 1].
+/// An immutable 64-bit signed integer, in the range `[-2^63, 2^63 - 1]`.
 /// Arithmetic operations may overflow in order to maintain this range.
 class Int64 implements IntX {
   // A 64-bit integer is represented internally as three non-negative