site: rollup resolve and onwarn improvements from sapper-template

site/rollup.config.js

@@ -13,6 +13,9 @@ const mode = process.env.NODE_ENV;
 const dev = mode === 'development';
 const legacy = !!process.env.SAPPER_LEGACY_BUILD;
 
+const onwarn = (warning, onwarn) => (warning.code === 'CIRCULAR_DEPENDENCY' && /[/\\]@sapper[/\\]/.test(warning.message)) || onwarn(warning);
+const dedupe = importee => importee === 'svelte' || importee.startsWith('svelte/');
+
 export default {
 	client: {
 		input: config.client.input(),
@@ -28,7 +31,10 @@ export default {
 				hydratable: true,
 				emitCss: true
 			}),
-			resolve(),
+			resolve({
+				browser: true,
+				dedupe
+			}),
 			commonjs(),
 			json(),
 
@@ -53,6 +59,7 @@ export default {
 				module: true
 			})
 		],
+		onwarn
 	},
 
 	server: {
@@ -67,7 +74,9 @@ export default {
 				generate: 'ssr',
 				dev
 			}),
-			resolve(),
+			resolve({
+				dedupe
+			}),
 			commonjs(),
 			json()
 		],
@@ -78,6 +87,7 @@ export default {
 				require('module').builtinModules || Object.keys(process.binding('natives'))
 			)
 		],
+		onwarn
 	},
 
 	serviceworker: {