Thanks for the contribution - would you add a "quick" test for this? The test method itself should be quick (empty string, non-empty string, maybe mutate a StringBuilder to be empty, then non-empty, then empty again), but setting up the test is going to be a little irritating to add since the test won't compile with Java 11, it will need to have super-source.

• Start with a class like com.google.gwt.emultest.java15.lang.StringTest (can copy from the java11.lang.StringTest for copyright header, superclass), but it will need a special runTest method like Java17Test has to only run with a high enough source level.
• The test method itself needs to be a stub with no implementation, since it wouldn't compile otherwise.
• Then, make a copy of the class, with no runTest, and with the stub populated, in somewhere like user/test-super/com/google/gwt/dev/jjs/super/com/google/gwt/emultest/java15/lang/StringTest.java. This will be supersourced, so your IDE may not see it properly, and will correctly not be compiled in Java 11.

Ok, will have a look when some free time is at hand,