Chore: Increase Runtime coverage

Source/DafnyCore/Backends/BoogieExtractor.cs

@@ -187,7 +187,7 @@ public override void VisitMethod(Method method) {
     }
 
     private QKeyValue? GetKeyValues(IToken tok, Attributes attributes) {
-      Boogie.QKeyValue kv = null;
+      QKeyValue? kv = null;
       var extractAttributes = Attributes.FindAllExpressions(attributes, AttributeAttribute);
       if (extractAttributes != null) {
         if (extractAttributes.Count == 0) {

Source/DafnyDriver/Commands/VerifyCommand.cs

@@ -98,7 +98,7 @@ public static async Task<int> HandleVerification(DafnyOptions options) {
             var extractedProgram = BoogieExtractor.Extract(resolution.ResolvedProgram);
             engine.PrintBplFile(options.BoogieExtractionTargetFile, extractedProgram, true, pretty: true);
           } catch (ExtractorError extractorError) {
-            options.OutputWriter.WriteLine($"Boogie axiom extraction error: {extractorError.Message}");
+            await options.OutputWriter.WriteLineAsync($"Boogie axiom extraction error: {extractorError.Message}");
             return 1;
           }
         }

Source/DafnyRuntime/AssemblyInfo.cs

@@ -0,0 +1,2 @@
+using System.Runtime.CompilerServices;
+[assembly: InternalsVisibleTo("DafnyRuntime.Tests")]

Source/DafnyRuntime/DafnyRuntime.cs

@@ -1280,8 +1280,8 @@ public override int Count {
   internal class ConcatSequence<T> : Sequence<T> {
     // INVARIANT: Either left != null, right != null, and elmts's underlying array == null or
     // left == null, right == null, and elmts's underlying array != null
-    private volatile ISequence<T> left, right;
-    private ImmutableArray<T> elmts;
+    internal volatile ISequence<T> left, right;
+    internal ImmutableArray<T> elmts;
     private readonly int count;
 
     internal ConcatSequence(ISequence<T> left, ISequence<T> right) {
@@ -1311,7 +1311,7 @@ public override int Count {
       }
     }
 
-    private ImmutableArray<T> ComputeElements() {
+    internal ImmutableArray<T> ComputeElements() {
       // Traverse the tree formed by all descendants which are ConcatSequences
       var ansBuilder = ImmutableArray.CreateBuilder<T>(count);
       var toVisit = new Stack<ISequence<T>>();

Source/DafnyRuntime/DafnyRuntime.csproj

@@ -25,7 +25,9 @@
   <ItemGroup>
     <Folder Include="DafnyRuntimeJava\build\libs" />
   </ItemGroup>
-
+   <ItemGroup>
+    <InternalsVisibleTo Include="$(MSBuildProjectName).Tests" />
+  </ItemGroup>
   <ItemGroup>
     <Content Update="DafnyRuntimeGo-gomod\dafny\dafny.go">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>