DAS: Simplify == code to use one expression

The code used to generate:

```
bool operator ==(Object other) {
  if (Other is X) {
    return a && b && c;
  }
  return false;
}
```

which is just simplified to the one expression:

```
bool operator ==(Object other) =>
  other is X && a && b && c;
```

Change-Id: I0d1a80b57598bd8e840c0cd99304ace64f4cd786
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/462740
Commit-Queue: Samuel Rawlins <[email protected]>
Auto-Submit: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: srawlins