[dart2wasm] Add program split constraints support to dart2wasm

This will allow a customer to pass
`--read-program-split=<constraints>.json` to the dart2wasm
compiler, which will then make use of the constraints to reduce the
number of wasm modules emitted.

We parametrize the dart2js program split builder code to allow
re-using of this code with Kernel AST. The changes to
`import_set.dart` we copy from dart2js to dart2wasm's implementation.

We also add testing support that partitions an application and
dumps the partitioning into expectation files:

* `pkg/dart2wasm/test/deferred_loading/partition_tests`: Custom tests
  for dart2wasm containing the Dart code as well as partitioning.

* `pkg/dart2wasm/test/deferred_loading/partition_tests_dart2js`:
  Partition expectation files for the existing tests in
  `pkg/compiler/test/custom_split/data/<testname>/main.dart`.

For the dart2js tests we dump normal partitioning and partitioning
under the constraints.

Change-Id: I9648899e45435d2a5ceb78126fb62ff00aae692d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/480921
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>