Fix broken tests & x86 edge case detection

- Update test csproj to include missing polyfills
- Fix net461 test compilation failures

src/libraries/System.Text.Encodings.Web/src/System/Text/Encodings/Web/SpanUtility.cs

@@ -239,10 +239,11 @@ public static bool TryWriteUInt64LittleEndian(Span<byte> span, int offset, ulong
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private static bool AreValidIndexAndLength(int spanRealLength, int requestedOffset, int requestedLength)
         {
+            // Logic here is copied from Span<T>.Slice.
             if (IntPtr.Size == 4)
             {
                 if ((uint)requestedOffset > (uint)spanRealLength) { return false; }
-                if ((uint)(spanRealLength - requestedLength) > (uint)requestedOffset) { return false; }
+                if ((uint)requestedLength > (uint)(spanRealLength - requestedOffset)) { return false; }
             }
             else
             {

src/libraries/System.Text.Encodings.Web/tests/AllowedBmpCodePointsBitmapTests.cs

@@ -27,7 +27,7 @@ public void Allow_Forbid_ZigZag()
         {
             // Arrange - we'll use BoundedMemory in this test to guard against
             // out-of-bounds accesses on the bitmap instance.
-            var boundedMem = BoundedMemory.Allocate<AllowedBmpCodePointsBitmap>(1);
+            using var boundedMem = BoundedMemory.Allocate<AllowedBmpCodePointsBitmap>(1);
             boundedMem.Span.Clear();
             ref var bitmap = ref boundedMem.Span[0];
 
@@ -112,7 +112,7 @@ public void IsCodePointAllowed_NonBmpCodePoints_ReturnsFalse()
         {
             // Arrange - we'll use BoundedMemory in this test to guard against
             // out-of-bounds accesses on the bitmap instance.
-            var boundedMem = BoundedMemory.Allocate<AllowedBmpCodePointsBitmap>(1);
+            using var boundedMem = BoundedMemory.Allocate<AllowedBmpCodePointsBitmap>(1);
             ref var bitmap = ref boundedMem.Span[0];
 
             Assert.False(bitmap.IsCodePointAllowed(0x10000)); // start of supplementary plane

src/libraries/System.Text.Encodings.Web/tests/AsciiByteMapTests.cs

@@ -26,7 +26,7 @@ public void MapEntries_ZigZag()
         {
             // Arrange - we'll use BoundedMemory in this test to guard against
             // out-of-bounds accesses on the byte map instance.
-            var boundedMem = BoundedMemory.Allocate<AsciiByteMap>(1);
+            using var boundedMem = BoundedMemory.Allocate<AsciiByteMap>(1);
             boundedMem.Span.Clear();
             ref var byteMap = ref boundedMem.Span[0];
 
@@ -62,7 +62,7 @@ public void TryLookup_NonAsciiCodePoints_ReturnsFalse()
         {
             // Arrange - we'll use BoundedMemory in this test to guard against
             // out-of-bounds accesses on the bitmap instance.
-            var boundedMem = BoundedMemory.Allocate<AsciiByteMap>(1);
+            using var boundedMem = BoundedMemory.Allocate<AsciiByteMap>(1);
             ref var byteMap = ref boundedMem.Span[0];
 
             Assert.False(byteMap.TryLookup(new Rune(128), out _)); // start of non-ASCII

src/libraries/System.Text.Encodings.Web/tests/System.Text.Encodings.Web.Tests.csproj

@@ -43,6 +43,7 @@
   </ItemGroup>
   <ItemGroup Condition="'$(TargetFramework)' == 'net461'">
     <Compile Include="..\src\Polyfills\System.Text.Rune.netstandard20.cs" Link="Polyfills\System.Text.Rune.netstandard20.cs" />
+    <Compile Include="..\src\System\Text\Encodings\Web\ThrowHelper.cs" Link="System\Text\Encodings\Web\ThrowHelper.cs" />
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="System.Private.Runtime.UnicodeData" Version="$(SystemPrivateRuntimeUnicodeDataVersion)" ExcludeAssets="contentFiles" GeneratePathProperty="true" />

src/libraries/System.Text.Encodings.Web/tests/UnicodeTestHelpers.cs

@@ -9,7 +9,7 @@ namespace System.Text.Unicode
 {
     internal static class UnicodeTestHelpers
     {
-        private static Lazy<StrongBox<AllowedBmpCodePointsBitmap>> _lazyBitmap = new Lazy<StrongBox<AllowedBmpCodePointsBitmap>>(InitializeLazyBitmap());
+        private static Lazy<StrongBox<AllowedBmpCodePointsBitmap>> _lazyBitmap = new Lazy<StrongBox<AllowedBmpCodePointsBitmap>>(InitializeLazyBitmap);
 
         /// <summary>
         /// Returns a value stating whether a character is defined per the checked-in version