Configure rollup to treeshake unused modules

Otherwise rollup inserts `import 'react'` in case the module has
side-effects.

Author: necolas

.flowconfig

@@ -16,6 +16,7 @@ component_syntax=true
 module.name_mapper='^react-strict-dom$' -> '<PROJECT_ROOT>/packages/react-strict-dom/dist/native/index.js.flow'
 module.system.node.resolve_dirname=flow_modules
 module.system.node.resolve_dirname=node_modules
+react.runtime=automatic
 suppress_type=$FlowIssue
 suppress_type=$FlowFixMe
 

packages/react-strict-dom/tools/rollup.config.mjs

@@ -55,7 +55,10 @@ const webConfigs = [
       file: path.join(__dirname, '../dist/dom/index.js'),
       format: 'es'
     },
-    plugins: [...sharedPlugins]
+    plugins: [...sharedPlugins],
+    treeshake: {
+      moduleSideEffects: false,
+    },
   },
   // Runtime
   {
@@ -65,7 +68,10 @@ const webConfigs = [
       file: path.join(__dirname, '../dist/dom/runtime.js'),
       format: 'es'
     },
-    plugins: [...sharedPlugins]
+    plugins: [...sharedPlugins],
+    treeshake: {
+      moduleSideEffects: false,
+    },
   }
 ];