Fix some closure compile warnings in JS library code (#21385)

Author: cwoffenden

src/library_browser.js

@@ -138,7 +138,9 @@ var LibraryBrowser = {
 #endif
         var img = new Image();
         img.onload = () => {
+#if ASSERTIONS
           assert(img.complete, `Image ${name} could not be decoded`);
+#endif
           var canvas = /** @type {!HTMLCanvasElement} */ (document.createElement('canvas'));
           canvas.width = img.width;
           canvas.height = img.height;
@@ -302,8 +304,9 @@ var LibraryBrowser = {
       if (!ctx) return null;
 
       if (setInModule) {
+#if ASSERTIONS
         if (!useWebGL) assert(typeof GLctx == 'undefined', 'cannot set in module if GLctx is used, but we are a non-GL context that would replace it');
-
+#endif
         Module.ctx = ctx;
         if (useWebGL) GL.makeContextCurrent(contextHandle);
         Module.useWebGL = useWebGL;
@@ -783,8 +786,9 @@ var LibraryBrowser = {
       return onerror ? onerror() : undefined;
     }
 #endif
+#if ASSERTIONS
     assert(runDependencies === 0, 'async_load_script must be run when no other dependencies are active');
-
+#endif
     {{{ runtimeKeepalivePush() }}}
 
     var loadDone = () => {
@@ -911,8 +915,9 @@ var LibraryBrowser = {
    * @param {boolean=} noSetTiming
    */`,
   $setMainLoop: (browserIterationFunc, fps, simulateInfiniteLoop, arg, noSetTiming) => {
+#if ASSERTIONS
     assert(!Browser.mainLoop.func, 'emscripten_set_main_loop: there can only be one main loop function at once: call emscripten_cancel_main_loop to cancel the previous one before setting a new one with different parameters.');
-
+#endif
     Browser.mainLoop.func = browserIterationFunc;
     Browser.mainLoop.arg = arg;
 

src/library_html5.js

@@ -315,6 +315,7 @@ var LibraryHTML5 = {
   // Users can also add more special event targets, basically by just doing something like
   //    specialHTMLTargets["!canvas"] = Module.canvas;
   // (that will let !canvas map to the canvas held in Module.canvas).
+  $specialHTMLTargets__docs: '/** @type {Object} */',
 #if ENVIRONMENT_MAY_BE_WORKER || ENVIRONMENT_MAY_BE_NODE || ENVIRONMENT_MAY_BE_SHELL || PTHREADS
   $specialHTMLTargets: "[0, typeof document != 'undefined' ? document : 0, typeof window != 'undefined' ? window : 0]",
 #else

src/library_idbfs.js

@@ -16,7 +16,9 @@ addToLibrary({
       if (typeof indexedDB != 'undefined') return indexedDB;
       var ret = null;
       if (typeof window == 'object') ret = window.indexedDB || window.mozIndexedDB || window.webkitIndexedDB || window.msIndexedDB;
+#if ASSERTIONS
       assert(ret, 'IDBFS used, but indexedDB not supported');
+#endif
       return ret;
     },
     DB_VERSION: 21,