Document --enable-asserts in dart2js's help, and remove --checked.

Closes #32648

Change-Id: I0d4cc8dd7117909b484be4119c0f5e7627cf3f85
Reviewed-on: https://dart-review.googlesource.com/48708
Reviewed-by: Kevin Moore <kevmoo@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>

pkg/compiler/lib/src/dart2js.dart

@@ -588,12 +588,12 @@ Supported options:
   -o <file>, --out=<file>
     Generate the output into <file>.
 
-  -c, --enable-checked-mode, --checked
-    Insert runtime type checks and enable assertions (checked mode).
-
   -m, --minify
     Generate minified output.
 
+  --enable-asserts
+    Enable assertions.
+
   -h, /h, /?, --help
     Display this message (add -v for information about all options).