Updates the test_fixtures rule to support a custom kernel output file
name. Improves rule documentation.

As of the migration to Dart 2, it has been necessary to compile Dart to
kernel prior to execution. The embedder currently requires that the
resulting kernel file be named `kernel_blob.bin` and be located at the
root of the assets directory passed to the embedder API.

This patch updates the test_fixtures build rule to perform a kernel
compile using frontend_server, outputting `kernel_blob.bin` to
`fixtures/test_target_name` directory, and updates the embedder
unittests to specify the kernel file rather than the Dart source file.

Since the kernel compiler requires a `main()` function to be defined, it
also updates `simple_main.dart` from runtime_unittests to define
`main()` rather than `simple_main()`.

This also updates all existing sub-targets to be testonly.

This relands commit ac9e521a, which was
reverted in commit 49411258. Rather than
running as prebuilt_dart_action, we use dart_action to ensure the
frontend snapshot it compatible with the VM on which it's executed.

This reverts commit ac9e521a.

This broke dynamic release mode builds of
//flutter/runtime:runtime_fixtures_kernel (likely all product-mode
builds).

Compile embedder unit test Dart to kernel

As of the migration to Dart 2, it has been necessary to compile Dart to
kernel prior to execution. The embedder currently requires that the
resulting kernel file be named `kernel_blob.bin` and be located at the
root of the assets directory passed to the embedder API.

This patch updates the test_fixtures build rule to perform a kernel
compile using frontend_server, outputting `kernel_blob.bin` to
`fixtures/test_target_name` directory, and updates the embedder
unittests to specify the kernel file rather than the Dart source file.

Since the kernel compiler requires a `main()` function to be defined, it
also updates `simple_main.dart` from runtime_unittests to define
`main()` rather than `simple_main()`.

This also updates all existing sub-targets to be testonly.