[Fix] Add missing selectros for NSExpression.

Added the missing static factory methods and the missing property. In
order to give a clean API a new flag was added to the NSExpression class
to track if the Block property does return a block or a null ptr. The
idea is to avoid user from seeing an obj-c exception.

This commit fixes bug #35012:

https://bugzilla.xamarin.com/show_bug.cgi?id=35012

The evaluation of the NSExpression and the defition of the
NSExpressionHandler have also been fixed since both should be using
NSObjects.

src/Foundation/NSExpression.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace XamCore.Foundation {
+
+	public partial class NSExpression {
+		bool hasFunction = false;
+
+		public static NSExpression FromFunction (NSExpressionHandler target, NSExpression[] parameters)
+		{
+			// NSExpression is a class cluster. The only subclass that supports the Block property is the one
+			// created by this static method. We set the flag so that the user does not get an obj-c exception
+			// when using the property and instead get a null ptr is returned.
+			var exp = _FromFunction (target, parameters);
+			exp.hasFunction = true;
+			return exp;
+		}
+
+		public NSExpressionHandler Block {
+			get {
+				if (hasFunction)
+					return this._Block;
+				return null;
+			}
+		} 
+	}
+}

src/foundation.cs

@@ -3152,8 +3152,7 @@ public interface NSException : NSCoding, NSCopying {
 		string[] CallStackSymbols { get; }
 	}
 
-	public delegate void NSExpressionHandler (NSObject evaluatedObject, NSExpression [] expressions, NSMutableDictionary context);
-
+	public delegate NSObject NSExpressionHandler (NSObject evaluatedObject, NSExpression [] expressions, NSMutableDictionary context);
 	[BaseType (typeof (NSObject))]
 	// Objective-C exception thrown.  Name: NSInvalidArgumentException Reason: *** -predicateFormat cannot be sent to an abstract object of class NSExpression: Create a concrete instance!
 	[DisableDefaultCtor]
@@ -3173,9 +3172,12 @@ public interface NSExpression : NSSecureCoding, NSCopying {
 		[Static, Export ("expressionForFunction:arguments:")]
 		NSExpression FromFunction (string name, NSExpression[] parameters);
 
+		[Static, Export ("expressionWithFormat:")]
+		NSExpression FromFormat (string expressionFormat);
+
 		[Static, Export ("expressionWithFormat:argumentArray:")]
-		NSExpression FromFormat (string format, NSExpression [] parameters);
-		
+		NSExpression FromFormat (string format, NSObject [] parameters);
+
 		//+ (NSExpression *)expressionForAggregate:(NSArray *)subexpressions; 
 		[Static, Export ("expressionForAggregate:")]
 		NSExpression FromAggregate (NSExpression [] subexpressions);
@@ -3196,8 +3198,8 @@ public interface NSExpression : NSSecureCoding, NSCopying {
 		[Static, Export ("expressionForFunction:selectorName:arguments:")]
 		NSExpression FromFunction (NSExpression target, string name, NSExpression[] parameters);
 
-		[Static, Export ("expressionForBlock:arguments:")]
-		NSExpression FromFunction (NSExpressionHandler target, NSExpression[] parameters);
+		[Internal, Static, Export ("expressionForBlock:arguments:")]
+		NSExpression _FromFunction (NSExpressionHandler target, NSExpression[] parameters);
 
 		[Since (7,0), Mavericks]
 		[Static]
@@ -3258,8 +3260,11 @@ public interface NSExpression : NSSecureCoding, NSCopying {
 		[Export ("falseExpression")]
 		NSExpression FalseExpression { get; }
 
+		[Internal, Export ("expressionBlock")]
+		NSExpressionHandler _Block { get; }
+
 		[Export ("expressionValueWithObject:context:")]
-		NSExpression ExpressionValueWithObject (NSObject object1, NSMutableDictionary context);
+		NSObject ExpressionValueWithObject ([NullAllowed] NSObject object1, [NullAllowed] NSMutableDictionary context);
 	}
 
 	[iOS (8,0)][Mac (10,10, onlyOn64 : true)] // Not defined in 32-bit

src/frameworks.sources

@@ -623,6 +623,7 @@ FOUNDATION_SOURCES = \
 	Foundation/NSDirectoryEnumerator.cs \
 	Foundation/NSDistributedNotificationCenter.cs \
 	Foundation/NSErrorException.cs \
+	Foundation/NSExpression.cs \
 	Foundation/NSExtension.cs \
 	Foundation/NSFastEnumerationState.cs \
 	Foundation/NSFastEnumerator.cs \