Lazy magic marshaling types map initialization (#2126)

Initialize the types map in `Prefill` method, filled by the
linker. Also pass the types count to the dictionary constructor to
improve performance and optimize memory allocation.

An example of `MagicRegistrationMap::Prefill` method, updated by the
linker, with the new code:

    using System.Collections.Generic;

    private static void Prefill ()
    {
    	MagicRegistrationMap.typesMap = new Dictionary<string, int> (5);
    	MagicRegistrationMap.typesMap ["xatemplateaot.MainActivity"] = 0;
    	MagicRegistrationMap.typesMap ["Android.Runtime.UncaughtExceptionHandler"] = 1;
    	MagicRegistrationMap.typesMap ["Android.Views.View+IOnClickListenerImplementor"] = 2;
    	MagicRegistrationMap.typesMap ["Java.Lang.Thread+RunnableImplementor"] = 3;
    	MagicRegistrationMap.typesMap ["Java.Interop.TypeManager+JavaTypeManager"] = 4;
    }

Author: radekdoulik

src/Mono.Android/Android.Runtime/AndroidRuntime.cs

@@ -291,14 +291,12 @@ static void Prefill ()
 
 			static MagicRegistrationMap ()
 			{
-				typesMap = new Dictionary<string, int> ();
-
 				Prefill ();
 			}
 
 			static public bool Filled {
 				get {
-					return typesMap.Count > 0;
+					return typesMap != null && typesMap.Count > 0;
 				}
 			}
 

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/MonoDroidMarkStep.cs

@@ -200,17 +200,23 @@ void UpdateMagicPrefill (TypeDefinition magicType)
 				return;
 
 			var typeDictionary = GetType ("mscorlib", "System.Collections.Generic.Dictionary`2");
+			var ctorDictionary = GetMethod (typeDictionary, ".ctor", new string[] { "System.Int32" });
 			var methodSetItem = GetMethod (typeDictionary, "set_Item", new string[] { "TKey", "TValue" });
 			var genericTypeDictionary = new GenericInstanceType (typeDictionary);
 			genericTypeDictionary.GenericArguments.Add (GetType ("mscorlib", "System.String"));
 			genericTypeDictionary.GenericArguments.Add (GetType ("mscorlib", "System.Int32"));
 
+			var genericMethodDictionaryCtor = CreateGenericMethodReference (ctorDictionary, genericTypeDictionary);
 			var genericMethodDictionarySetItem = CreateGenericMethodReference (methodSetItem, genericTypeDictionary);
 			var importedMethodSetItem = magicType.Module.ImportReference (genericMethodDictionarySetItem);
 
 			var instructions = methodPrefill.Body.Instructions;
 			instructions.Clear ();
 
+			instructions.Add (CreateLoadArraySizeOrOffsetInstruction (marshalTypes.Count));
+			instructions.Add (Instruction.Create (OpCodes.Newobj, magicType.Module.ImportReference (genericMethodDictionaryCtor)));
+			instructions.Add (Instruction.Create (OpCodes.Stsfld, fieldTypesMap));
+
 			int idx = 0;
 
 			foreach (var type in marshalTypes) {