Here is the output from my perf testing tool currently in review that
shows a significant difference in csc.exe after the change:

```
// * Detailed results *
PlaceholderBenchmarkRunner.PlaceholderMethod: Job-TGYVCW(Toolchain=Perf.ExternalProcessToolchain, LaunchCount=0, RunStrategy=Monitoring, TargetCount=25, WarmupCount=0) [Commit=HEAD^]
Runtime = ; GC =
Mean = 4.8018 s, StdErr = 0.0215 s (0.45%); N = 25, StdDev = 0.1073 s
Min = 4.6717 s, Q1 = 4.7297 s, Median = 4.7892 s, Q3 = 4.8438 s, Max = 5.1998 s
IQR = 0.1141 s, LowerFence = 4.5585 s, UpperFence = 5.0149 s
ConfidenceInterval = [4.7214 s; 4.8822 s] (CI 99.9%), Margin = 0.0804 s (1.67% of Mean)
Skewness = 1.87, Kurtosis = 7.98

PlaceholderBenchmarkRunner.PlaceholderMethod: Job-TGYVCW(Toolchain=Perf.ExternalProcessToolchain, LaunchCount=0, RunStrategy=Monitoring, TargetCount=25, WarmupCount=0) [Commit=HEAD]
Runtime = ; GC =
Mean = 3.6409 s, StdErr = 0.0650 s (1.78%); N = 25, StdDev = 0.3249 s
Min = 3.4381 s, Q1 = 3.5510 s, Median = 3.5781 s, Q3 = 3.6189 s, Max = 5.1720 s
IQR = 0.0679 s, LowerFence = 3.4491 s, UpperFence = 3.7209 s
ConfidenceInterval = [3.3975 s; 3.8842 s] (CI 99.9%), Margin = 0.2433 s (6.68% of Mean)
Skewness = 4.16, Kurtosis = 19.76

Total time: 00:07:11 (431.13 sec)

// * Summary *

BenchmarkDotNet=v0.10.9, OS=Windows 10 Redstone 1 (10.0.14393)
Processor=Intel Xeon CPU E5645 2.40GHz, ProcessorCount=24
Frequency=2337892 Hz, Resolution=427.7358 ns, Timer=TSC
.NET Core SDK=2.1.1-preview-007094
  [Host] : .NET Core 2.0.0 (Framework 4.6.00001.0), 64bit RyuJIT

Toolchain=Perf.ExternalProcessToolchain  LaunchCount=0  RunStrategy=Monitoring
TargetCount=25  WarmupCount=0

            Method | Commit |    Mean |    Error |   StdDev | Rank |
------------------ |------- |--------:|---------:|---------:|-----:|
 PlaceholderMethod |   HEAD | 3.641 s | 0.2433 s | 0.3249 s |    1 |
 PlaceholderMethod |  HEAD^ | 4.802 s | 0.0804 s | 0.1073 s |    2 |

// * Legends *
  Commit : Value of the 'Commit' parameter
  Mean   : Arithmetic mean of all measurements
  Error  : Half of 99.9% confidence interval
  StdDev : Standard deviation of all measurements
  Rank   : Relative position of current benchmark mean among all benchmarks (Arabic style)
  1 s    : 1 Second (1 sec)

```

Fixes #22815

This reverts commit 27dc60f5.

* Compare lines instead of using AssertXml when validating PDBs.

AssertXml doesn't validate ordering.

* Reorder expected PDB XML nodes to match actual order

* Update to the latest Microsoft.DiaSymReader.* packages.

* Enable embedding sources to Windows PDBs

made diagnostic engine to verify span given to diagnostic from analyzers. and added explicit check in tagger for out of range

* Ref assemblies make a best effort to emit with determinism
* Fixing CopyRefAssembly message output
* CopyRefAssembly logs when input isn't ref assembly
* CopyRefAssembly does assembly redirect.
* Compact error codes for C# 7.1

* Update versions

* Improve diagnostics

Added null IFormatProvider for moved messaged
Modified tests

https://github.com/dotnet/roslyn/issues/11512

* refonly and refout on command-line and msbuild
* corresponding change to Emit API
* Metadata-only assembly use `throw null` bodies
* Can no longer produce netmodule with metadata only

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems?id=233669.

Add a version header to Object-Streams.  This way we can tell, up front if the persistence version has changed, and if we can read the data properly or not.

This reverts commit 7dd78ae8, reversing
changes made to 449f5586.

We have seen two big analyzer packages (CodeCracker and RefactoringEssentials) end up with registering async actions, which can crash VS from the async continuations.
See [this](https://github.com/code-cracker/code-cracker/issues/375#issue-87474837) for more details. Now we prevent this by disallowing analyzers to register async callbacks.

Fixes https://github.com/dotnet/roslyn/issues/13120

* 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.