Core: Warn and fill jQuery.isArray

Fixes jquery#220

Author: dmethvin

src/core.js

@@ -140,4 +140,11 @@ if ( jQueryVersionSince( "3.3.0" ) ) {
 		},
 		"jQuery.isWindow() is deprecated"
 	);
+
+	migrateWarnFunc( jQuery, "isArray",
+		function( a ) {
+			return Array.isArray( a );
+		},
+		"jQuery.isArray is deprecated; use Array.isArray"
+	);
 }

test/core.js

@@ -430,9 +430,20 @@ QUnit[ jQueryVersionSince( "3.3.0" ) ? "test" : "skip" ]( "jQuery.type (warn)",
 
 } );
 
+QUnit[ jQueryVersionSince( "3.3.0" ) ? "test" : "skip" ]( "jQuery.isArray", function( assert ) {
+	assert.expect( 4 );
+
+	expectWarning( assert, "isArray", 1, function() {
+		assert.equal( jQuery.isArray( [] ), true, "empty array" );
+		assert.equal( jQuery.isArray( "" ), false, "empty string" );
+		assert.equal( jQuery.isArray( jQuery().toArray() ), true, "toArray" );
+	} );
+
+} );
+
 TestManager.runIframeTest( "old pre-3.0 jQuery", "core-jquery2.html",
 	function( assert, jQuery, window, document, log ) {
 		assert.expect( 1 );
 
 		assert.ok( /jQuery 3/.test( log ), "logged: " + log );
-	} );
+} );

warnings.md

@@ -241,3 +241,8 @@ See jQuery-ui [commit](https://github.com/jquery/jquery-ui/commit/c0093b599fcd58
 **Cause:** The `jQuery.cssProps` property is a public but undocumented object that allows CSS properties with one name to be mapped into another name. It was used for legacy browsers like IE8 that used non-standard names. This object is no longer used inside jQuery since all supported browsers now use the standard CSS property names.
 
 **Solution:** Remove any uses of `jQuery.cssProps` in application code.
+
+#### JQMIGRATE: jQuery.isArray is deprecated; use Array.isArray
+**Cause:** Older versions of JavaScript made it difficult to determine if a particular object was a true Array, so jQuery provided a cross-browser function to do the work. The browsers supported by jQuery 3.0 all provide a standard method for this purpose.
+
+**Solution:** Replace any calls to `jQuery.isArray` with `Array.isArray`.