[CFE/kernel/vm] Incremental compiler/serializer and metadata

This CL:
* Makes the incremental compiler copy over the metadata just created for
  the libraries compiled into the Component returned.
* Preserves metadata when serializing via the incremental serializer.
* Fixes loading a multi-component with metadata.
* Lets the VM copy all metadata in "_combinePendingDeltas"
  when there is only one component to "combine".

It is generally my view, that to do better, metadata has to be redone
(among other things likely has to live on the Library instead).

https://github.com/dart-lang/sdk/issues/39302

With this CL:

```
rm foo.dill ; tools/sdks/dart-sdk/bin/dart pkg/frontend_server/bin/frontend_server_starter.dart --no-print-incremental-dependencies --target=vm --sdk-root=out/ReleaseX64 --platform=vm_platform_strong.dill --output-dill=foo.dill --packages=.dart_tool/package_config.json pkg/compiler/lib/src/dart2js.dart
tools/sdks/dart-sdk/bin/dart pkg/vm/bin/dump_kernel.dart foo.dill foo.dump
grep "@vm.call-site-attributes" foo.dump | wc -l
```

(i.e. a one-shot compile) prints 9500.

```
rm foo.dill ; tools/sdks/dart-sdk/bin/dart pkg/frontend_server/bin/frontend_server_starter.dart --incremental --no-incremental-serialization --no-print-incremental-dependencies --target=vm --sdk-root=out/ReleaseX64 --platform=vm_platform_strong.dill --output-dill=foo.dill --packages=.dart_tool/package_config.json pkg/compiler/lib/src/dart2js.dart
tools/sdks/dart-sdk/bin/dart pkg/vm/bin/dump_kernel.dart foo.dill foo.dump
grep "@vm.call-site-attributes" foo.dump | wc -l
```

(i.e. an incremental compile but without incremental serialization) prints 9500.

and

```
rm foo.dill ; tools/sdks/dart-sdk/bin/dart pkg/frontend_server/bin/frontend_server_starter.dart --incremental --no-print-incremental-dependencies --target=vm --sdk-root=out/ReleaseX64 --platform=vm_platform_strong.dill --output-dill=foo.dill --packages=.dart_tool/package_config.json pkg/compiler/lib/src/dart2js.dart
tools/sdks/dart-sdk/bin/dart pkg/vm/bin/dump_kernel.dart foo.dill foo.dump
grep "@vm.call-site-attributes" foo.dump | wc -l
```

(i.e. an incremental compile with incremental serialization) prints 9500.

If not deleting the dill file first (which will then initialize from it)
metadata might be different though.
If there are no changes and the incremental serializer was and is used
we should still get `9530` (because the bytes are just copied), but
otherwise we might get different results because it's not really
possible to otherwise pick-and-choose and/or merge the metadata.

TEST=Existing tests that it didn't get worse.

Change-Id: I19d530357e7e2a174d7408dbbf0ed7e0248a5b70
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/358444
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>