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These helpers will allow building a graph, modifying it and runnign more compiler passes on them as well as easily building assembly stubs for testing purposes. TEST=Adds test helpers Change-Id: I3bfef21e1370a7c99c9bbbe4f123211f6d9fe964 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/233781 Reviewed-by: Slava Egorov <vegorov@google.com> Commit-Queue: Martin Kustermann <kustermann@google.com>