reintro unkeyed each block iterations as necessary

src/generators/dom/visitors/EachBlock.js

@@ -304,12 +304,21 @@ function unkeyed ( generator, block, state, node, snippet, { create_each_block,
 
 	if ( condition !== '' ) {
 		const forLoopBody = node._block.hasUpdateMethod ?
+			node._block.hasIntroMethod ?
 				deindent`
 					if ( ${iterations}[${i}] ) {
 						${iterations}[${i}].update( changed, ${params}, ${each_block_value}, ${each_block_value}[${i}], ${i} );
 					} else {
 						${iterations}[${i}] = ${create_each_block}( ${params}, ${each_block_value}, ${each_block_value}[${i}], ${i}, ${block.component} );
-					${iterations}[${i}].${mountOrIntro}( ${parentNode}, ${anchor} );
+					}
+					${iterations}[${i}].intro( ${parentNode}, ${anchor} );
+				` :
+				deindent`
+					if ( ${iterations}[${i}] ) {
+						${iterations}[${i}].update( changed, ${params}, ${each_block_value}, ${each_block_value}[${i}], ${i} );
+					} else {
+						${iterations}[${i}] = ${create_each_block}( ${params}, ${each_block_value}, ${each_block_value}[${i}], ${i}, ${block.component} );
+						${iterations}[${i}].mount( ${parentNode}, ${anchor} );
 					}
 				` :
 			deindent`

src/shared/dom.js

@@ -18,7 +18,7 @@ export function detachBetween ( before, after ) {
 
 export function destroyEach ( iterations, detach, start ) {
 	for ( var i = start; i < iterations.length; i += 1 ) {
-		iterations[i].destroy( detach );
+		if ( iterations[i] ) iterations[i].destroy( detach );
 	}
 }