[Core] Continue on errors when resolving assembly references

If a PackageReference could not be resolved then no assembly
references would be given to the type system from the DotNetProject's
GetReferencedAssemblies. Whilst the NuGet restore may fail the
project.assets.json file is created and reference information is
available. The ResolvePackageAssets target was failing which then
stopped the ResolveAssemblyReferencesDesignTime target from being run.
Visual Studio on Windows avoids this problem by setting the
ContinueOnError property to ErrorAndContinue. This allows all the
targets to run even if some of them fail. It also allows references
to be returned and provided to the type system service for the
PackageReferences that can be determined.

Fixes VSTS #998324 - NuGet Package Restore causes BCLs to not be
defined/imported

Author: mrward

main/src/core/MonoDevelop.Core/MonoDevelop.Projects/DotNetProject.cs

@@ -1137,6 +1137,9 @@ async Task<List<AssemblyReference>> RunResolveAssemblyReferencesTarget (Configur
 				context.LogVerbosity = MSBuildVerbosity.Quiet;
 				context.GlobalProperties.SetValue ("Silent", true);
 				context.GlobalProperties.SetValue ("DesignTimeBuild", true);
+				// Even though some targets may fail it may still be possible for the main resolve targets to return
+				// information so we set ContinueOnError. This matches VS on Windows behaviour.
+				context.GlobalProperties.SetValue ("ContinueOnError", "ErrorAndContinue");
 
 				var result = await RunTargetInternal (monitor, "ResolveAssemblyReferencesDesignTime;ResolveProjectReferencesDesignTime", configuration, context);
 				refs = result.Items.Select (i => new AssemblyReference (i.Include, i.Metadata)).ToList ();

main/tests/MonoDevelop.Core.Tests/MonoDevelop.Projects/ProjectTests.cs

@@ -1108,6 +1108,31 @@ public async Task ImplicitlyExpandDesignTimeFacades ()
 			}
 		}
 
+		[Test]
+		public async Task UnknownNuGetPackageReferenceId_DesignTimeBuilds ()
+		{
+			FilePath solFile = Util.GetSampleProject ("UnknownPackageReference", "UnknownPackageReference.sln");
+			CreateNuGetConfigFile (solFile.ParentDirectory);
+
+			// Run restore but do not check result since this will fail.
+			using var process = Process.Start (new ProcessStartInfo {
+				FileName = "nuget",
+				Arguments = $"restore -DisableParallelProcessing \"{solFile}\"",
+				RedirectStandardError = true,
+				RedirectStandardOutput = true,
+				UseShellExecute = false
+			});
+			Assert.IsTrue (process.WaitForExit (120000), "Timeout restoring NuGet packages.");
+
+			using (var sol = (Solution)await Services.ProjectService.ReadWorkspaceItem (Util.GetMonitor (), solFile)) {
+				var project = sol.GetAllProjects ().Single () as DotNetProject;
+				var refs = (await project.GetReferencedAssemblies (ConfigurationSelector.Default)).ToArray ();
+
+				Assert.IsTrue (refs.Any ());
+				Assert.IsTrue (refs.Any (r => r.FilePath.FileName == "Newtonsoft.Json.dll"));
+			}
+		}
+
 		/// <summary>
 		/// Clear all other package sources and just use the main NuGet package source when
 		/// restoring the packages for the project tests.

main/tests/test-projects/UnknownPackageReference/UnknownPackageReference.csproj

@@ -0,0 +1,11 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>netstandard1.5</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Newtonsoft.Json" Version="10.0.1" />
+    <PackageReference Include="UnknownPackageIdThatDoesNotExistAnywhere" Version="1.0.3" />
+   </ItemGroup>
+</Project>

main/tests/test-projects/UnknownPackageReference/UnknownPackageReference.sln

@@ -0,0 +1,17 @@
+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio 15
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "UnknownPackageReference", "UnknownPackageReference.csproj", "{EEC2E74C-F54E-4E9E-B0A6-4E7A0D71A9A9}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{EEC2E74C-F54E-4E9E-B0A6-4E7A0D71A9A9}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{EEC2E74C-F54E-4E9E-B0A6-4E7A0D71A9A9}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{EEC2E74C-F54E-4E9E-B0A6-4E7A0D71A9A9}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{EEC2E74C-F54E-4E9E-B0A6-4E7A0D71A9A9}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+EndGlobal