Add diagnostics messages and update Changelog related to changed DOM element lookup rules.

Author: juj

ChangeLog.md

@@ -16,6 +16,13 @@ full changeset diff at the end of each section.
 Current Trunk
 -------------
  - Remove deprecated Pointer_stringify (use UTF8ToString instead). See #8011
+ - Added a new option -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=1 that
+   changes the lookup semantics of DOM elements in html5.h event handler
+   callback and WebGL context creation. New behavior is to use CSS selector
+   strings to look up DOM elements over the old behavior, which was somewhat
+   ad hoc constructed rules around default Emscripten uses. The old behavior
+   will be deprecated and removed in the future. Build with -s ASSERTIONS=1
+   to get diagnostics messages related to this transition.
 
 v1.38.26: 02/04/2019
 --------------------

src/library_html5.js

@@ -284,7 +284,14 @@ var LibraryJSEvents = {
 
   _findEventTarget__deps: ['_maybeCStringToJsString', '_specialEventTargets'],
   _findEventTarget: function(target) {
-    return __specialEventTargets[target] || document.querySelector(__maybeCStringToJsString(target));
+    var domElement = __specialEventTargets[target] || document.querySelector(__maybeCStringToJsString(target));
+#if ASSERTIONS
+    // TODO: Remove this check in the future, or move it to some kind of debugging mode, because it may be perfectly fine behavior
+    // for one to query an event target to test if any DOM element with given CSS selector exists. However for a migration period
+    // from old lookup over to new, it is very useful to get diagnostics messages related to a lookup failing.
+    if (!domElement) err('No DOM element was found with CSS selector "' + __maybeCStringToJsString(target) + '"');
+#endif
+    return domElement;
   },
 
 #if OFFSCREENCANVAS_SUPPORT
@@ -313,7 +320,7 @@ var LibraryJSEvents = {
   _findEventTarget__deps: ['_specialEventTargets'],
   _findEventTarget: function(target) {
 #if ASSERTIONS
-    warnOnce('Rules for selecting event targets in HTML5 API are changing: instead of using document.getElementById() that only can refer to elements by their DOM ID, new event target selection mechanism uses the more flexible function document.querySelector() that can look up element names, classes, and complex CSS selectors. Build with -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=1 to change to the new lookup rules.');
+    warnOnce('Rules for selecting event targets in HTML5 API are changing: instead of using document.getElementById() that only can refer to elements by their DOM ID, new event target selection mechanism uses the more flexible function document.querySelector() that can look up element names, classes, and complex CSS selectors. Build with -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=1 to change to the new lookup rules. See https://github.com/emscripten-core/emscripten/pull/7977 for more details.');
 #endif
     try {
       // The sensible "default" target varies between events, but use window as the default