Revert "[vm] Build dart2native dependencies with the normal "create_sdk" target"

This reverts commit a88a328b7f5afb887baee0e5d7c2db681e52efd6.

Reason for revert: Auto roller hasn't rolled previous changes from
Dart yet. Those changes updated DEPS for icu/boringssl. We have to
wait with this CL until the DEPS changes have been done in the engine.

Original change's description:
> [vm] Build dart2native dependencies with the normal "create_sdk" target
> 
> This removes special logic for creating the `dart-sdk` we distribute
> which used to build release and product mode and copied some binaries
> from the latter into the former, before the SDK was actuallly ready to
> test and distribute.
> 
> This changes the GN build rules to build the necessary
> gen_snapshot/dart_precompiled_runtime product binaries during the normal
> release build.
> 
> Normally during --mode=product builds the global build config in
> //build/config/BUILDCONFIG.gn will set `-fvisibility=false`.
> 
> => Doing so results in much smaller binaries - because only explicitly
>    exported symbols are visible, the rest can be tree shaken by the linker.
> 
> Since we are building --mode=release, the `-fvisibility=false` will not
> be set. In order to set the flag for the 2 special product-mode binaries
> we need to add -fvisibility=hidden manually, in:
> 
>   * dart_product_config: Which is used for compiling VM sources.
>   * 3rd party double-conversion library
>   * 3rd party boringssl library
>   * 3rd party icu library
> 
> The upstream CLs are:
> 
>   * BoringSSL: https://dart-review.googlesource.com/c/boringssl_gen/+/150482
>   * ICU: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/2236407
> 
> Issue https://github.com/dart-lang/sdk/issues/42230
> 
> Change-Id: I3e47664d9fadb9ed1ad033bb17d46e769442f741
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150524
> Commit-Queue: Martin Kustermann <kustermann@google.com>
> Reviewed-by: Alexander Thomas <athom@google.com>
> Reviewed-by: Zach Anderson <zra@google.com>

TBR=kustermann@google.com,zra@google.com,athom@google.com,asiva@google.com

Change-Id: Ied0f2f5b58dceb2b59bc2ac6e11e5fd717c8885e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151382
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>