Assembly Resolver perf improvement (#1218)

* Adding null in resolvedAssemblies dictionary for assemblies which couldnt be resolved.
It is giving ~3.1 seconds perf improvement for 10k xunit tests from vstest.console during execution.

* Removed extra white spaces

* PR comments

* PR comments

src/Microsoft.TestPlatform.Common/Utilities/AssemblyResolver.cs

@@ -123,6 +123,7 @@ private Assembly OnResolve(object sender, AssemblyResolveEventArgs args)
                         EqtTrace.Info("AssemblyResolver: {0}: Failed to create assemblyName. Reason:{1} ", args.Name, ex);
                     }
 
+                    this.resolvedAssemblies[args.Name] = null;
                     return null;
                 }
 
@@ -175,9 +176,15 @@ private Assembly OnResolve(object sender, AssemblyResolveEventArgs args)
                         }
                     }
                 }
-            }
 
-            return null;
+                if (EqtTrace.IsInfoEnabled)
+                {
+                    EqtTrace.Info("AssemblyResolver: {0}: Failed to load assembly.", args.Name);
+                }
+
+                this.resolvedAssemblies[args.Name] = null;
+                return null;
+            }
         }
 
         /// <summary>