Sensitive object logging redaction

A while back, I described this on the [wrong repo](https://github.com/flutter/flutter/issues/132822). I'm finally getting around to putting it in the right place; here. :)

Consider the following scenario: You've got a data class, such as the following.

```dart
class Credentials {
final String username;
final String password;

Credentials({
required this.username,
required this.password,
});

Map toJson() => {'username': username, 'password': password};

@override
String toString() => toJson().toString();
}
```

The following is the _current_ behavior:

```dart
final Credentials myCredentials = Credentials(
username: "hans",
password: "password",
);

print(myCredentials);
```

Output:
`I/flutter (10392): {username: hans, password: password}`

What I'm proposing is a _new_ keyword, such as `sensitive`, that would look something like this:

```dart
class Credentials {
final String username;
final String password;

Credentials({
required this.username,
sensitive required this.password,
});
...
```

So, at a system level when you try to log/print the parameter, you'd see something like this:

`I/flutter (10392): {username: hans, password: }`

Start by reviewing the proposal and its examples in this issue, including the suggested sensitive keyword and the logging output. No repository files, tests, or entry points are identified; done would require an agreed language design for syntax and redaction behavior.

Written by the indexing model from the issue text.