Make the CLI run generate/output in serial

As a temporary workaround for concurrency issues in the plugin
driver.

See issue #3174

cli/run/build.ts

@@ -66,9 +66,10 @@ export default function build(
 				});
 			}
 
-			return Promise.all(outputOptions.map(output => bundle.write(output) as Promise<any>)).then(
-				() => bundle
-			);
+			return outputOptions.reduce(
+				(prev, output) => prev.then(() => bundle.write(output) as Promise<any>),
+				Promise.resolve()
+			).then(() => bundle)
 		})
 		.then((bundle: RollupBuild | null) => {
 			if (!silent) {

test/cli/samples/emit-file-multiple-dirs/.tern-port

@@ -0,0 +1 @@
+46665

test/cli/samples/emit-file-multiple-dirs/_config.js

@@ -0,0 +1,4 @@
+module.exports = {
+	description: 'writes files emitted by plugins in different output dirs',
+	command: 'rollup -c'
+};

test/cli/samples/emit-file-multiple-dirs/_expected/dist1/main.js

@@ -0,0 +1,3 @@
+'use strict';
+
+assert.equal( ANSWER, 42 );

test/cli/samples/emit-file-multiple-dirs/_expected/dist1/myfile

@@ -0,0 +1 @@
+abc

test/cli/samples/emit-file-multiple-dirs/_expected/dist2/main.js

@@ -0,0 +1 @@
+assert.equal( ANSWER, 42 );

test/cli/samples/emit-file-multiple-dirs/_expected/dist2/myfile

@@ -0,0 +1 @@
+abc

test/cli/samples/emit-file-multiple-dirs/main.js

@@ -0,0 +1 @@
+assert.equal( ANSWER, 42 );

test/cli/samples/emit-file-multiple-dirs/rollup.config.js

@@ -0,0 +1,18 @@
+export default {
+  input: "main.js",
+  output: [
+    {
+      dir: "_actual/dist1",
+      format: "cjs"
+    },
+    {
+      dir: "_actual/dist2",
+      format: "esm"
+    }
+  ],
+  plugins: [{
+    generateBundle() {
+      this.emitFile({type: "asset", fileName: "myfile", source: "abc"})
+    }
+  }]
+}