In STRICT mode, only define assert when ASSERTIONS is set

This matches the behaviour of MINIMAL_RUNTIME, and supports the practice
of not including asserts in release builds.

Also remove the closure externs for `assert` so that closure can
correctly error out end `assert` is not defined.

Helps with emscripten-core#20504

Author: sbc100

src/closure-externs/node-externs.js

@@ -57,11 +57,6 @@ wss._socket.remoteAddress;
 */
 wss._socket.remotePort;
 
-/**
- * @suppress {duplicate}
- */
-var assert;
-
 /**
  * @suppress {duplicate}
  */

src/preamble.js

@@ -75,6 +75,11 @@ var ABORT = false;
 // but only when noExitRuntime is false.
 var EXITSTATUS;
 
+#if ASSERTIONS || !STRICT
+// In STRICT mode, we only define assert() when ASSERTIONS is set.  i.e. we
+// don't define it at all in release modes.  This matches the behaviour of
+// MINIMAL_RUNTIME.
+// TODO(sbc): Make this the default even without STRICT enabled.
 /** @type {function(*, string=)} */
 function assert(condition, text) {
   if (!condition) {
@@ -88,6 +93,7 @@ function assert(condition, text) {
 #endif
   }
 }
+#endif
 
 #if ASSERTIONS
 // We used to include malloc/free by default in the past. Show a helpful error in