Fix build dependencies.

src/Microsoft.AspNetCore.Razor/Tokenizer/CSharpTokenizer.cs

@@ -91,7 +91,12 @@ namespace Microsoft.AspNetCore.Razor.Tokenizer
                 case CSharpTokenizerState.AtSymbolAfterRazorCommentBody:
                     return AtSymbolAfterRazorCommentBody();
                 default:
+#if NET451
+                    // No Debug.Fail
                     Debug.Fail("Invalid TokenizerState");
+#else
+                    Debug.Assert(false, "Invalid TokenizerState");
+#endif
                     return default(StateResult);
             }
         }

src/Microsoft.AspNetCore.Razor/Tokenizer/HtmlTokenizer.cs

@@ -77,7 +77,12 @@ namespace Microsoft.AspNetCore.Razor.Tokenizer
                 case HtmlTokenizerState.AtSymbolAfterRazorCommentBody:
                     return AtSymbolAfterRazorCommentBody();
                 default:
+#if NET451
+                    // No Debug.Fail
                     Debug.Fail("Invalid TokenizerState");
+#else
+                    Debug.Assert(false, "Invalid TokenizerState");
+#endif
                     return default(StateResult);
             }
         }

src/Microsoft.AspNetCore.Razor/project.json

@@ -22,7 +22,8 @@
     "dotnet5.4": {
       "dependencies": {
         "System.IO.FileSystem": "4.0.1-*",
-        "System.Linq": "4.0.2-*",
+        "System.Linq": "4.1.0-*",
+        "System.Runtime.Extensions": "4.1.0-*",
         "System.Security.Cryptography.Algorithms": "4.0.0-*",
         "System.Threading.Thread": "4.0.0-*"
       }