[dart2wasm] Allow dartPrint function to intercept print()s from Dart

Instead of directly printing all `print()`s from Dart to the console,
we allow the prints to be intercepted - by specifying a `dartPrint()`
function.

=> This will fix our test infrastructure, which will now recognize
async tests as being asynchronous and waits until the async test
is actually done with testing.

=> This will also make the test infrastructure report all prints
of a test when the test fails (it reports DOM, prints, various other
events)

=> Test failure output of `tools/test.py -n dart2js-*-{firefox,chrome}`
will become actually useful, containing printed errors & stack traces.

This is similar to what dart2js does in

    sdk/lib/_internal/js_runtime/lib/js_primitives.dart

Related Issue: https://github.com/dart-lang/sdk/issues/54973

Change-Id: Ibd935fd2d0b15d1b6e444214c4556a65034957dd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/354100
Reviewed-by: Ömer Ağacan <omersa@google.com>