CoreFx work

This changes the following:

- Changed the fallback encoding for EncodedStringText to Latin1 instead
of code page 1252.  The latter is not supported on CoreCLR so using the
more general Latin1.
- Change the VB compiler code base to be CoreCLR compatible as well.

src/Compilers/Core/Desktop/EncodedStringText.cs

@@ -23,7 +23,7 @@ internal static class EncodedStringText
         /// from <see cref="SourceText.From(Stream, Encoding, SourceHashAlgorithm, bool)"/> in two ways:
         /// 1. It attempts to minimize allocations by trying to read the stream into a byte array.
         /// 2. If <paramref name="defaultEncoding"/> is null, it will first try UTF8 and, if that fails, it will
-        ///    try extended ASCII (code page 1252).
+        ///    try extended ASCII (code page Latin1).
         /// </summary>
         /// <param name="stream">The stream containing encoded text.</param>
         /// <param name="defaultEncoding">
@@ -57,7 +57,7 @@ internal static SourceText Create(Stream stream, Encoding defaultEncoding = null
 
             try
             {
-                return Decode(stream, defaultEncoding ?? Encoding.GetEncoding(codepage: 1252), checksumAlgorithm, throwIfBinaryDetected: detectEncoding);
+                return Decode(stream, defaultEncoding ?? Encoding.GetEncoding(name: "Latin1"), checksumAlgorithm, throwIfBinaryDetected: detectEncoding);
             }
             catch (DecoderFallbackException e)
             {

src/Compilers/VisualBasic/Desktop/CommandLine/CommandLineParser.vb

@@ -1051,12 +1051,12 @@ lVbRuntimePlus:
             End If
 
             ' Locate default 'mscorlib.dll' or 'System.Runtime.dll', if any.
-            Dim defaultCoreLibraryReference As CommandLineReference? = LoadCoreLibraryReference(sdkPaths, baseDirectory)
+            Dim defaultCoreLibraryReference As CommandLineReference? = LoadCoreLibraryReference(sdkPaths, baseDirectory, sdkDirectory)
 
             ' If /nostdlib is not specified, load System.dll
             ' Dev12 does it through combination of CompilerHost::InitStandardLibraryList and CompilerProject::AddStandardLibraries.
             If Not noStdLib Then
-                Dim systemDllPath As String = FindFileInSdkPath(sdkPaths, "System.dll", baseDirectory)
+                Dim systemDllPath As String = FindFileInSdkPath(sdkPaths, "System.dll", baseDirectory, sdkDirectory)
                 If systemDllPath Is Nothing Then
                     AddDiagnostic(diagnostics, ERRID.WRN_CannotFindStandardLibrary1, "System.dll")
                 Else
@@ -1069,7 +1069,7 @@ lVbRuntimePlus:
             ' Add reference to 'Microsoft.VisualBasic.dll' if needed
             If includeVbRuntimeReference Then
                 If vbRuntimePath Is Nothing Then
-                    Dim msVbDllPath As String = FindFileInSdkPath(sdkPaths, "Microsoft.VisualBasic.dll", baseDirectory)
+                    Dim msVbDllPath As String = FindFileInSdkPath(sdkPaths, "Microsoft.VisualBasic.dll", baseDirectory, sdkDirectory)
                     If msVbDllPath Is Nothing Then
                         AddDiagnostic(diagnostics, ERRID.ERR_LibNotFound, "Microsoft.VisualBasic.dll")
                     Else
@@ -1209,7 +1209,7 @@ lVbRuntimePlus:
                 }
         End Function
 
-        Private Function LoadCoreLibraryReference(sdkPaths As List(Of String), baseDirectory As String) As CommandLineReference?
+        Private Function LoadCoreLibraryReference(sdkPaths As List(Of String), baseDirectory As String, sdkDirectory As String) As CommandLineReference?
             ' Load Core library in Dev11:
             ' Traditionally VB compiler has hard-coded the name of mscorlib.dll. In the Immersive profile the
             ' library is called System.Runtime.dll. Ideally we should get rid of the dependency on the name and
@@ -1219,8 +1219,8 @@ lVbRuntimePlus:
             ' There is an extra check to only pick an assembly with no other assembly refs. This is so that is an 
             ' user drops a user-defined binary called System.runtime.dll into the fx directory we still want to pick 
             ' mscorlib. 
-            Dim msCorLibPath As String = FindFileInSdkPath(sdkPaths, "mscorlib.dll", baseDirectory)
-            Dim systemRuntimePath As String = FindFileInSdkPath(sdkPaths, "System.Runtime.dll", baseDirectory)
+            Dim msCorLibPath As String = FindFileInSdkPath(sdkPaths, "mscorlib.dll", baseDirectory, sdkDirectory)
+            Dim systemRuntimePath As String = FindFileInSdkPath(sdkPaths, "System.Runtime.dll", baseDirectory, sdkDirectory)
 
             If systemRuntimePath IsNot Nothing Then
                 If msCorLibPath Is Nothing Then
@@ -1252,9 +1252,9 @@ lVbRuntimePlus:
             Return Nothing
         End Function
 
-        Private Function FindFileInSdkPath(sdkPaths As List(Of String), fileName As String, baseDirectory As String) As String
+        Private Function FindFileInSdkPath(sdkPaths As List(Of String), fileName As String, baseDirectory As String, sdkDirectory As String) As String
             For Each path In sdkPaths
-                Dim absolutePath = FileUtilities.ResolveRelativePath(If(path, RuntimeEnvironment.GetRuntimeDirectory()), baseDirectory)
+                Dim absolutePath = FileUtilities.ResolveRelativePath(If(path, sdkDirectory), baseDirectory)
                 If absolutePath IsNot Nothing Then
                     Dim filePath = PathUtilities.CombineAbsoluteAndRelativePaths(absolutePath, fileName)
                     If File.Exists(filePath) Then