simpler code for case where there is only one outro in a block

src/compile/dom/Block.ts

@@ -262,7 +262,7 @@ export default class Block {
 						${outroing} = true;
 						${hasIntros && `${introing} = false;`}
 
-						var #outros = ${this.outros};
+						${this.outros > 1 && `var #outros = ${this.outros};`}
 
 						${this.builders.outro}
 					},

src/compile/nodes/Element.ts

@@ -701,18 +701,18 @@ export default class Element extends Node {
 			const fn = `%transitions-${intro.name}`;
 
 			block.builders.intro.addBlock(deindent`
-				#component.root._aftercreate.push(function() {
+				#component.root._aftercreate.push(() => {
 					if (!${name}) ${name} = @wrapTransition(#component, ${this.var}, ${fn}, ${snippet}, true, null);
-					${name}.run(true, function() {
+					${name}.run(true, () => {
 						#component.fire("intro.end", { node: ${this.var} });
 					});
 				});
 			`);
 
 			block.builders.outro.addBlock(deindent`
-				${name}.run(false, function() {
+				${name}.run(false, () => {
 					#component.fire("outro.end", { node: ${this.var} });
-					if (--#outros === 0) #outrocallback();
+					${block.outros > 1 ? `if (--#outros === 0) #outrocallback();` : `#outrocallback();`}
 					${name} = null;
 				});
 			`);
@@ -736,9 +736,9 @@ export default class Element extends Node {
 				}
 
 				block.builders.intro.addBlock(deindent`
-					#component.root._aftercreate.push(function() {
+					#component.root._aftercreate.push(() => {
 						${introName} = @wrapTransition(#component, ${this.var}, ${fn}, ${snippet}, true, null);
-						${introName}.run(true, function() {
+						${introName}.run(true, () => {
 							#component.fire("intro.end", { node: ${this.var} });
 						});
 					});
@@ -757,9 +757,9 @@ export default class Element extends Node {
 				// group) prior to their removal from the DOM
 				block.builders.outro.addBlock(deindent`
 					${outroName} = @wrapTransition(#component, ${this.var}, ${fn}, ${snippet}, false, null);
-					${outroName}.run(false, function() {
+					${outroName}.run(false, () => {
 						#component.fire("outro.end", { node: ${this.var} });
-						if (--#outros === 0) #outrocallback();
+						${block.outros > 1 ? `if (--#outros === 0) #outrocallback();` : `#outrocallback();`}
 					});
 				`);
 			}