* Implement source link feature

* Fix CommonCompiler to open source link stream, add E2E tests

* Reduce code duplication across VB and C#

Revert "Revert "Revert "Revert "*DON'T MERGE INTO MASTER* Hide IOperation API behind a feature flag""""

This reverts commit d4d502e4.

This reverts commit c6a1c2c8.

* Tests: fix minasync Task<T> to derive from Task

* Tests: provide MinAsyncCorlibRef

This combines the async features of minasync with mincorlib to produce
a minimum unversioned corlib with async stubs.

* Compilation: use System.Object from target corlib

When creating a script compilation without an explicit return type,
System.Object was being resolved via reflection from the host.

This resulted in an implicit dependency of a script compilation on the
host corlib, even if a different corlib was specified as a reference
for the compilation (e.g. Xamarin.iOS).

Fix this by using System.Object as defined in the corlib resovled
for the compilation.

* Tests: fix minasync Task<T> to derive from Task

* Tests: provide MinAsyncCorlibRef

This combines the async features of minasync with mincorlib to produce
a minimum unversioned corlib with async stubs.

* Compilation: use System.Object from target corlib

When creating a script compilation without an explicit return type,
System.Object was being resolved via reflection from the host.

This resulted in an implicit dependency of a script compilation on the
host corlib, even if a different corlib was specified as a reference
for the compilation (e.g. Xamarin.iOS).

Fix this by using System.Object as defined in the corlib resovled
for the compilation.

Ensure that we generated a CompilationCompletedEvent for compilations with no source files (example an empty VSIX projects).

Ensure that we generated a CompilationCompletedEvent for compilations with no source files (example an empty VSIX projects).

This reverts commit a96d7825.

This reverts commit 92ff3f8d.

This reverts commit b03e8478.

https://github.com/dotnet/roslyn/commit/b03e84783806dc48f240e56188e6473323dc3ba1

The flag name is "IOperation". Register opeartion actions and calling
SemanticModel.GetOperation while the feature is disabled will throw an
InvalidOperationException. Also log a non-fatal-watson when
SemanticModel.GetOperation crashes.

Sometimes called "fake sign" or "OSS sign" public signing is including
the public key in an output assembly and setting the "signed" flag, but
not actually signing the assembly with a private key. This is useful for
open source projects where people want to build assemblies which are
compatible with the released "fully signed" assemblies, but don't have
access to the private key used to sign the assemblies. Since almost no
consumers actually need to check if the assembly is fully signed, these
publicly built assemblies are useable in almost every scenario that the
fully signed one would be used in.

This PR implements support only for C# -- VB will be added soon. If
being used at the command line, the /publicsign flag can be passed to
csc and the /keyfile flag can specify the public key. Unlike fully
signing, a full key pair encoded in the SNK file format is not currently
supported. When using /publicsign, just the public key must be in the
/keyfile file.

When using the API, the public key can be passed directly using the
CryptoPublicKey CompilationOption.