ts: Convert `emojisets` to TypeScript.

Declared types for '*.png' modules and '!style-loader?*' modules in
`assets.d.ts` and used them in `emojisets.ts`.

tools/test-js-with-node

@@ -86,7 +86,7 @@ EXEMPT_FILES = make_set(
         "web/src/dropdown_widget.js",
         "web/src/echo.js",
         "web/src/emoji_picker.js",
-        "web/src/emojisets.js",
+        "web/src/emojisets.ts",
         "web/src/favicon.ts",
         "web/src/feedback_widget.ts",
         "web/src/flatpickr.js",

web/src/assets.d.ts

@@ -7,3 +7,15 @@ declare module "*.ttf" {
     const url: string;
     export default url;
 }
+
+declare module "*.png" {
+    const url: string;
+    export default url;
+}
+
+// Declare the style loader for CSS files.  This is used in the
+// `import` statements in the `emojisets.ts` file.
+declare module "!style-loader?*" {
+    const css: {use: () => void; unuse: () => void};
+    export default css;
+}

web/src/emojisets.ts

@@ -10,7 +10,12 @@ import google_blob_css from "!style-loader?injectType=lazyStyleTag!css-loader!..
 import google_css from "!style-loader?injectType=lazyStyleTag!css-loader!../generated/emoji-styles/google-sprite.css";
 import twitter_css from "!style-loader?injectType=lazyStyleTag!css-loader!../generated/emoji-styles/twitter-sprite.css";
 
-const emojisets = new Map([
+type EmojiSet = {
+    css: {use: () => void; unuse: () => void};
+    sheet: string;
+};
+
+const emojisets = new Map<string, EmojiSet>([
     ["google", {css: google_css, sheet: google_sheet}],
     ["google-blob", {css: google_blob_css, sheet: google_blob_sheet}],
     ["twitter", {css: twitter_css, sheet: twitter_sheet}],
@@ -18,15 +23,20 @@ const emojisets = new Map([
 
 // For `text` emoji set we fallback to `google` emoji set
 // for displaying emojis in emoji picker and typeahead.
-emojisets.set("text", emojisets.get("google"));
+emojisets.set("text", emojisets.get("google")!);
 
-let current_emojiset;
+let current_emojiset: EmojiSet | undefined;
 
-export async function select(name) {
+export async function select(name: string): Promise<void> {
     const new_emojiset = emojisets.get(name);
     if (new_emojiset === current_emojiset) {
         return;
     }
+
+    if (!new_emojiset) {
+        throw new Error("Unknown emojiset " + name);
+    }
+
     await new Promise((resolve, reject) => {
         const sheet = new Image();
         sheet.addEventListener("load", resolve);
@@ -45,8 +55,8 @@ export async function select(name) {
     current_emojiset = new_emojiset;
 }
 
-export function initialize() {
-    select(user_settings.emojiset);
+export function initialize(): void {
+    void select(user_settings.emojiset);
 
     // Load the octopus image in the background, so that the browser
     // will cache it for later use.  Note that we hardcode the octopus