Move package bundle functionality into SummaryBaseTestStrategy.
Move package bundle functionality into SummaryBaseTestStrategy.

The methods addBundle and createPackageBundle were previously in the
derived class SummaryLinkerTestStrategy, but there's no reason they
have to be there; they don't rely on any functionality that's specific
to linker tests.  Moving them will allow additional summary tests to
make use of package bundles.

Change-Id: I988147c76e6dcfef5ef0f000d66b1639b4b3ecf7
Reviewed-on: https://dart-review.googlesource.com/c/79280
Auto-Submit: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>
Commit-Queue: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>

dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk