Fix failing tests:

- Don't do analysis for speculative attributes
- Manually call TryGetThisParameter, rather than MethodThisParameter which can throw

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -261,7 +261,7 @@ protected override ImmutableArray<PendingBranch> Scan(ref bool badRegion)
                 _returnTypesOpt.Clear();
             }
             this.Diagnostics.Clear();
-            ParameterSymbol methodThisParameter = MethodThisParameter;
+            ((MethodSymbol)_symbol).TryGetThisParameter(out var methodThisParameter);
             this.State = TopState();                   // entry point is reachable
             this.regionPlace = RegionPlace.Before;
             EnterParameters();                               // with parameters assigned
@@ -293,7 +293,8 @@ protected override ImmutableArray<PendingBranch> Scan(ref bool badRegion)
             BoundAttribute attribute,
             DiagnosticBag diagnostics)
         {
-            if (attribute.Constructor is null)
+            if (attribute.Constructor is null ||
+                !compilation.SyntaxTrees.Contains(attribute.SyntaxTree))
             {
                 return;
             }