Add 'use strict' to generated JS when -s STRICT is used (#9265)

See #8317

emcc.py

@@ -1095,6 +1095,7 @@ def check(input_file):
 
     if shared.Settings.STRICT:
       shared.Settings.DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR = 1
+      shared.Settings.STRICT_JS = 1
 
     if AUTODEBUG:
       shared.Settings.AUTODEBUG = 1

src/settings.js

@@ -813,8 +813,12 @@ var LINKABLE = 0;
 //   * DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR is enabled
 //   * The C define EMSCRIPTEN is not defined (__EMSCRIPTEN__ always is, and
 //     is the correct thing to use).
+//   * STRICT_JS is enabled
 var STRICT = 0;
 
+// Add "use strict;" to generated JS
+var STRICT_JS = 0;
+
 // If set to 1, we will warn on any undefined symbols that are not resolved by
 // the library_*.js files. Note that it is common in large projects to not
 // implement everything, when you know what is not going to actually be called

src/shell.js

@@ -3,6 +3,10 @@
 // University of Illinois/NCSA Open Source License.  Both these licenses can be
 // found in the LICENSE file.
 
+#if STRICT_JS
+"use strict;"
+
+#endif
 #if SIDE_MODULE == 0
 // The Module object: Our interface to the outside world. We import
 // and export values on it. There are various ways Module can be used: