Harden ParseEmbeddedStatement

The consumers of ParseEmbeddedStatement depend on it having a non-null return.  Yet it directly returns the result of ParseStatementCore which can validly return null values in error conditions.  In the case that does happen return an empty statement with a diagnostic.

src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

@@ -7457,10 +7457,19 @@ private StatementSyntax ParseEmbeddedStatement(bool complexCheck)
                 statement = this.ParseStatementCore();
             }
 
+            // The consumers of embedded statements are expecting to receive a non-null statement yet there are 
+            // several error conditions that can lead ParseStatementCore to return null.  When that occurs 
+            // create an empty error Statement and return it to the caller.
+            if (statement == null)
+            {
+                Debug.Assert(CurrentToken.Kind != SyntaxKind.SemicolonToken);
+                statement = SyntaxFactory.EmptyStatement(EatToken(SyntaxKind.SemicolonToken));
+            }
+
             // An "embedded" statement is simply a statement that is not a labelled
             // statement or a declaration statement.  Parse a normal statement and post-
             // check for the error case.
-            switch (statement?.Kind)
+            switch (statement.Kind)
             {
                 case SyntaxKind.LabeledStatement:
                 case SyntaxKind.LocalDeclarationStatement:

src/Compilers/CSharp/Test/Syntax/Parsing/StatementParsingTests.cs

@@ -2623,6 +2623,40 @@ static void Test(int arg1, (byte, byte) arg2)
             Assert.Equal(false, tree.GetRoot().ContainsDiagnostics);
         }
 
+        [Fact]
+        [WorkItem(684860, "https://devdiv.visualstudio.com/DevDiv/_workitems/edit/266237")]
+        public void DevDiv266237()
+        {
+            var source = @"
+class Program
+{
+    static void Go()
+    {
+        using (var p = new P
+        {
+
+        }
+
+    protected override void M()
+    {
+
+    }
+}
+";
+
+            var tree = SyntaxFactory.ParseSyntaxTree(source, options: TestOptions.Regular);
+            tree.GetDiagnostics(tree.GetRoot()).Verify(
+                // (9,10): error CS1026: ) expected
+                //         }
+                CSharpTestBase.Diagnostic(ErrorCode.ERR_CloseParenExpected, "").WithLocation(9, 10),
+                // (9,10): error CS1002: ; expected
+                //         }
+                CSharpTestBase.Diagnostic(ErrorCode.ERR_SemicolonExpected, "").WithLocation(9, 10),
+                // (9,10): error CS1513: } expected
+                //         }
+                CSharpTestBase.Diagnostic(ErrorCode.ERR_RbraceExpected, "").WithLocation(9, 10));
+        }
+
         private sealed class TokenAndTriviaWalker : CSharpSyntaxWalker
         {
             public int Tokens;