During hot-restart, wait to run `main` when `pause_isolates_on_start` is true

[elliette]

During a hot-restart, if pause_isolates_on_start is true, DWDS waits for a call to resume before running the app's main method. This gives the debugging client time to re-set any breakpoints before the app is run.

When using RequireStrategy (flutter, webdev):

• dwds/web/reloader/require_restarter.dart waits for the readyToRunMain future before calling the main method

When using DdcStrategy (g3):

• The readyToRunMain future is converted to a promise and passed to pkg/dev_compiler/lib/js/ddc/ddc_module_loader.js (externally, different script internally), which then waits for it to resolve before calling the main method
• Note: requires https://dart-review.googlesource.com/c/sdk/+/355880 - question: is this script used in any configuration?

Work towards https://github.com/flutter/devtools/issues/7231

[nshahan]

Note: requires https://dart-review.googlesource.com/c/sdk/+/355880 - question: is this script used in any configuration?

Currently it is used by the expression compiler tests on the ddc test configurations in the SDK and it is also used by the benchmarking infra when running code with DDC. There is a very minor test in the SDK that runs on the benchmark-linux config that ensures we don't totally break the pieces the benchmark infra relies on.

We will be adding more configs and tests in the near future though.