[build] Fix various warnings (#812)

Update `Java.Interop.dll` to fix some nullable reference type warnings.

Update `Java.Interop.Localization.csproj` to add
`$(NeutralLanguage)`=en, which fixes:

	Warning CA1824: Mark assemblies with NeutralResourcesLanguageAttribute

This basically says that the "neutral" language within
`Java.Interop.Localization.dll` is also the English translation, so
we don't need to spend time probing for satellite resource assemblies
when the English language is requested.

Update `Java.Runtime.Environment.csproj` to add
`$(MSBuildWarningsAsMessages)`=NU1702, to suppress the following:

	Warning NU1702: ProjectReference 'D:\a\1\s\src\java-interop\java-interop.csproj'
	was resolved using '.NETFramework,Version=v4.7.2' instead of the project
	target framework '.NETStandard,Version=v2.0'. 
	This project may not be fully compatible with your project.

Author: jpobst

src/Java.Interop.Localization/Java.Interop.Localization.csproj

@@ -5,6 +5,7 @@
     <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
     <DefineConstants>INTERNAL_NULLABLE_ATTRIBUTES</DefineConstants>
+    <NeutralLanguage>en</NeutralLanguage>
   </PropertyGroup>
 
   <ItemGroup>

src/Java.Interop/Java.Interop/JavaArray.cs

@@ -30,7 +30,10 @@ public int Length {
 
 		[MaybeNull]
 		public abstract T this [int index] {
+			// I think this will be fixable in .NET5+ with support for "T?"
+#pragma warning disable CS8766 // Nullability of reference types in return type doesn't match implicitly implemented member (possibly because of nullability attributes).
 			get;
+#pragma warning restore CS8766
 			set;
 		}
 

src/Java.Interop/Java.Interop/JniEnvironment.Types.cs

@@ -180,7 +180,7 @@ public static void RegisterNatives (JniObjectReference type, JniNativeMethodRegi
 						var method  = m.Marshaler.Method;
 						Debug.WriteLine ($"JNIEnv::RegisterNatives() given a generic delegate type.  .NET Core doesn't like this.");
 						Debug.WriteLine ($"  Java: {m.Name}{m.Signature}");
-						Debug.WriteLine ($"  Marshaler Type={m.Marshaler.GetType ().FullName} Method={method.DeclaringType.FullName}.{method.Name}");
+						Debug.WriteLine ($"  Marshaler Type={m.Marshaler.GetType ().FullName} Method={method.DeclaringType!.FullName}.{method.Name}");
 					}
 				}
 #endif  // DEBUG && NETCOREAPP

src/Java.Interop/Java.Interop/JniRuntime.JniMarshalMemberBuilder.cs

@@ -87,7 +87,7 @@ public  Delegate                                                CreateMarshalToM
 			{
 				if (value == null)
 					throw new ArgumentNullException (nameof (value));
-				return CreateMarshalToManagedExpression (value.GetMethodInfo ()).Compile ();
+				return CreateMarshalToManagedExpression (value.GetMethodInfo ()!).Compile ();
 			}
 
 			public  abstract    LambdaExpression                            CreateMarshalToManagedExpression (MethodInfo method);

src/Java.Interop/Java.Interop/JniType.cs

@@ -14,7 +14,7 @@ namespace Java.Interop {
 
 	public sealed class JniType : IDisposable {
 
-		[return: NotNullIfNotNull ("name")]
+		[return: NotNullIfNotNull ("classFileData")]
 		public static unsafe JniType? DefineClass (string name, JniObjectReference loader, byte[] classFileData)
 		{
 			if (classFileData == null)

src/Java.Runtime.Environment/Java.Runtime.Environment.csproj

@@ -7,6 +7,7 @@
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
+    <MSBuildWarningsAsMessages>NU1702</MSBuildWarningsAsMessages>
   </PropertyGroup>
 
   <PropertyGroup>