control Rollup version via URL

src/app/main.js

@@ -119,7 +119,7 @@ if ( supported ) {
 			});
 
 			// save state as hash fragment
-			history.replaceState( {}, 'x', `/?shareable=${btoa( encodeURIComponent( JSON.stringify({ options, modules }) ) )}` );
+			history.replaceState( {}, 'x', `/?version=${rollup.VERSION}&shareable=${btoa( encodeURIComponent( JSON.stringify({ options, modules }) ) )}` );
 		})
 		.catch( error => {
 			output.set( 'error', error );

src/files/index.html

@@ -53,6 +53,32 @@
 		</div>
 	</main>
 
+	<!-- load selected version of Rollup, then the app -->
+	<script>
+		(function () {
+			function loadScript ( src ) {
+				return new Promise( ( fulfil, reject ) => {
+					var script = document.createElement( 'script' );
+					script.onload = fulfil;
+					script.onerror = reject;
+
+					script.src = src;
+
+					document.querySelector( 'head' ).appendChild( script );
+				});
+			}
+
+			var versionMatch = /version=([^&]+)/.exec( window.location.search );
+			var path = versionMatch ?
+				'https://unpkg.com/rollup@' + versionMatch[1] + '/dist/rollup.browser.js' :
+				'https://unpkg.com/rollup/dist/rollup.browser.js';
+
+			loadScript( path ).then( function () {
+				loadScript( 'app.js' );
+			});
+		}());
+	</script>
+
 	<!-- CodeMirror JS -->
 	<script src='/codemirror/lib/codemirror.js'></script>
 	<script src='/codemirror/mode/javascript/javascript.js'></script>
@@ -60,10 +86,6 @@
 	<!-- Ractive -->
 	<script src='/ractive/ractive.js'></script>
 
-	<!-- Rollup and app -->
-	<script src='https://unpkg.com/rollup/dist/rollup.browser.js'></script>
-	<script src='app.js'></script>
-
 	<!-- Google Analytics -->
 	<script>
 		(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){