Fix webidl_binder so it can't handle missing assert function

This can happen when ASSERTIONS is not set and we are in STRICT
mode or MINIMAL_RUNTIME mode.

See emscripten-core#20592

Author: sbc100

test/test_core.py

@@ -7669,6 +7669,7 @@ def test_embind_no_rtti_followed_by_rtti(self):
   def test_webidl(self, mode, allow_memory_growth):
     self.uses_es6 = True
     self.set_setting('WASM_ASYNC_COMPILATION', 0)
+    self.set_setting('STRICT')
     if self.maybe_closure():
       # avoid closure minified names competing with our test code in the global name space
       self.set_setting('MODULARIZE')

test/webidl/post.js

@@ -239,7 +239,7 @@ try {
 } catch (e) {}
 
 try {
-  s = new TheModule.StringUser('abc', 1);
+  var s = new TheModule.StringUser('abc', 1);
   s.Print(123, null); // Expects a string or a wrapped pointer
 } catch (e) {}
 

tools/webidl_binder.py

@@ -133,6 +133,15 @@ def build_constructor(name):
 mid_js += build_constructor('WrapperObject')
 
 mid_js += ['''
+/*
+ * For now, the webidl-generated code unconditionally depends on the `assert` function,
+ * but there are certain build modes where emscripten does not define this.
+ * TODO(sbc): Make the usage of assert conditional.
+ */
+if (typeof assert == "undefined") {
+  assert = (cond) => {}
+}
+
 /** @suppress {duplicate} (TODO: avoid emitting this multiple times, it is redundant)
     @param {*=} __class__ */
 function getCache(__class__) {