This is the first step in adding support for Razor compilation at build
time. Additional steps will complete this by adding tag helper
discovery, code generation and finally a call to CsC.
I want to get this in now to get the package into the build system and
to lay down the general infrastructure for testing.
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The strategy for testing here is to use checked in projects that have
everything Razor needs to compile code. We already have shims for the
APIs Razor uses by default in this repo and I'm using them in the
project.
The test infrastructure creates a temporary directory, copies the
project, and fixes up a few small things to cooperate with outputs we've
already built so that the test can use the new versions of Razor bits
built from this repo.
We can then call various MSBuild targets and verify the files on disk. I
envision tests that verify incrementalism as well as the basic E2E.
We will test the E2E in general in other places, since it's part of the
new default experience. This repo will test things at a higher level of
detail, but in slightly artifical scenarios.
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