Fix None being reported as not defined in type hints (microsoft#336)

Author: jakebailey

src/Analysis/Engine/Impl/Analyzer/ExpressionEvaluator.cs

@@ -150,7 +150,7 @@ public IAnalysisSet LookupAnalysisSetByName(Node node, string name, bool addRef
                         res = refs.Types;
                     } else {
                         // ... warn the user
-                        warn = true;
+                        warn = !(node is ConstantExpression); // Don't warn when None.
                     }
                 }
             }

src/Analysis/Engine/Test/LanguageServerTests.cs

@@ -363,6 +363,27 @@ public async Task DiagnosticsSettingChange() {
             }
         }
 
+        [TestMethod, Priority(0)]
+        public async Task TypeHintNoneDiagnostic() {
+            if (this is LanguageServerTests_V2) {
+                // No type hints in Python 2.
+                return;
+            }
+
+            var code = @"
+def f(b: None) -> None:
+    b: None
+";
+
+            var diags = new Dictionary<Uri, PublishDiagnosticsEventArgs>();
+            using (var s = await CreateServer((Uri)null, null, diags)) {
+                var u = await s.OpenDefaultDocumentAndGetUriAsync(code);
+
+                await s.WaitForCompleteAnalysisAsync(CancellationToken.None);
+                GetDiagnostics(diags, u).Should().BeEmpty();
+            }
+        }
+
         [TestMethod, Priority(0)]
         public async Task OnTypeFormattingLine() {
             using (var s = await CreateServer()) {