model: Implement dark-mode stream color swatches

[chrisbobbe]

Stacked atop #642 #654.

---

We added the light-mode color computations in #381.

We're not ready to use these yet, but the color computations are
unlikely to change before that time comes, and finding the right
ones is a chore that's good to get done.

Related: #95

[chrisbobbe]

Here is how I generated the "expected" values to put in the tests. (This comment is adapted from #381 (comment) ; this time for dark mode.)

Here's the code I added in the replit, just under ZULIP_ASSIGNMENT_COLORS. It duplicates some of the logic, where it wasn't exposed with a convenient helper function. But it can be checked against the implementation. → Actually, one color follows the web app instead of the replit; I've noted that in the code and also in the snippet below.

const dartColorIntFromHexString = (hexString) => {
  const lowercaseWithoutHash = hexString.substring(1).toLowerCase();
  switch (lowercaseWithoutHash.length) {
    case 6: // opaque (alpha omitted)
      return `0xff${lowercaseWithoutHash}`;
    case 8: // not opaque
      // convert rrggbbaa to aarrggbb
      return '0x'
        + lowercaseWithoutHash.substring(6, 8)
        + lowercaseWithoutHash.substring(0, 2);
    default:
      throw new Error();
  }
};

const dartCheckLine = (baseHex, expectedHex) => {
  return `runCheck(${dartColorIntFromHexString(baseHex)}, const Color(${dartColorIntFromHexString(expectedHex)}));\n`;
};

let iconOnPlainBackgroundResult = '';
let iconOnBarBackgroundResult = '';
let barBackgroundResult = '';
ZULIP_ASSIGNMENT_COLORS.forEach(color => {
  const correctedColor = correctStreamColor({ color });
  const barBackground = getRecipientBarColor({ color: correctStreamColor({ color }), darkMode: true });
  const iconOnPlainBackground = correctedColor;
  const iconOnBarBackground = correctedColor; // follows web, not replit

  iconOnPlainBackgroundResult += dartCheckLine(color, iconOnPlainBackground);
  iconOnBarBackgroundResult += dartCheckLine(color, iconOnBarBackground);
  barBackgroundResult += dartCheckLine(color, barBackground);
});

console.log(iconOnPlainBackgroundResult);
console.log(iconOnBarBackgroundResult);
console.log(barBackgroundResult);

[gnprice]

Thanks! Just small comments below. (Modulo the review I just left on #642.)

[gnprice]

use Flutter's clampDouble instead; see its docs

[gnprice]

This comment is all the same as in the light case, right?

Let's try to deduplicate comments that don't differ. It makes it easier to find the spots where there is new information in these comments (after reading the other version's comments), like for iconOnBarBackground.

[chrisbobbe]

Thanks for the review! Revision pushed, and now this is atop

• model: Implement StreamColorSwatch.lerp, copying ColorSwatch.lerp #654

instead of

• model [nfc]: Remove StreamColorSwatch; just use ColorSwatch<StreamColorVariant> #642

.

[gnprice]

Thanks for the revision! LGTM — merging.