Disable failing Mono test

https://github.com/mono/mono/issues/12603

src/Compilers/CSharp/Test/Symbol/Compilation/LoadDirectiveTests.cs

@@ -64,7 +64,7 @@ public void FileWithErrors()
             }
         }
 
-        [Fact]
+        [ConditionalFact(typeof(ClrOnly), Reason = "https://github.com/mono/mono/issues/12603")]
         public void FileThatCannotBeDecoded()
         {
             var code = "#load \"b.csx\"";

src/Compilers/Core/CodeAnalysisTest/EmbeddedTextTests.cs

@@ -244,7 +244,7 @@ public void FromSource_Precomputed()
             }
         }
 
-        [Fact]
+        [ConditionalFact(typeof(ClrOnly), Reason = "https://github.com/mono/mono/issues/12603")]
         public void FromBytes_EncodingFallbackCase()
         {
             var source = EncodedStringText.Create(new MemoryStream(new byte[] { 0xA9, 0x0D, 0x0A }), canBeEmbedded: true);

src/Compilers/Core/Portable/CommandLine/CommonCompiler.cs

@@ -369,7 +369,7 @@ internal static DiagnosticInfo ToFileReadDiagnostics(CommonMessageProvider messa
         {
             DiagnosticInfo diagnosticInfo;
 
-            if (e is FileNotFoundException || e.GetType().Name == "DirectoryNotFoundException")
+            if (e is FileNotFoundException || e is DirectoryNotFoundException)
             {
                 diagnosticInfo = new DiagnosticInfo(messageProvider, messageProvider.ERR_FileNotFound, filePath);
             }

src/Test/Utilities/Portable/Assert/ConditionalFactAttribute.cs

@@ -160,7 +160,23 @@ public class HasShiftJisDefaultEncoding : ExecutionCondition
 
     public class HasEnglishDefaultEncoding : ExecutionCondition
     {
-        public override bool ShouldSkip => Encoding.GetEncoding(0)?.CodePage != 1252;
+        public override bool ShouldSkip
+        {
+            get
+            {
+                try
+                {
+                    return Encoding.GetEncoding(0)?.CodePage != 1252;
+                }
+                catch (EntryPointNotFoundException)
+                {
+                    // Mono is throwing this exception on recent runs. Need to just assume false in this case while the
+                    // bug is tracked down. 
+                    // https://github.com/mono/mono/issues/12603
+                    return false;
+                }
+            }
+        }
 
         public override string SkipReason => "OS default codepage is not Windows-1252.";
     }