…otnet#8063)

* # This is a combination of 9 commits.
# This is the 1st commit message:

ref -> mutable in more places in the compiler

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# Update dependencies from https://github.com/dotnet/arcade build 20191229.1
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# - Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19629.1

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# Update dependencies from https://github.com/dotnet/arcade build 20191230.1
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# - Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19630.1

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# Update dependencies from https://github.com/dotnet/arcade build 20191231.1
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# - Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.19631.1

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# Update dependencies from https://github.com/dotnet/arcade build 20200101.1
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# - Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20051.1

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# Update dependencies from https://github.com/dotnet/arcade build 20191216.5 (dotnet#8079)
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# - Microsoft.DotNet.Arcade.Sdk - 1.0.0-beta.19616.5

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# dispose fsi at the end of a scripting session (dotnet#8084)
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# Added static link tests and extended CompilerAssert (dotnet#8101)
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# * Changed CompilerAssert to static class. Added Compile/Execute methods that take a Compilation description. Added static link tests
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# * Hiding compilation description internals
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# * Added another test to check for sanity
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# * Making a few optional parameters
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# * Hiding internals of CompilationReference

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# Parameterize product version (dotnet#8031)
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# * Parameterize Product details
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# * fcs
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# * Repack pkgdef

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