Cleanly handle case here no .packages file exists

Author: kevmoo

webdev/bin/webdev.dart

@@ -27,6 +27,13 @@ Future main(List<String> args) async {
       }
     }
 
+    exitCode = ExitCode.config.code;
+  } on FileSystemException catch (e) {
+    print(yellow.wrap('Could not run in the current directory.'));
+    print(e.message);
+    if (e.path != null) {
+      print('  ${e.path}');
+    }
     exitCode = ExitCode.config.code;
   }
 }

webdev/lib/src/command/build_runner_command_base.dart

@@ -11,6 +11,8 @@ import 'package:stack_trace/stack_trace.dart';
 
 import '../pubspec.dart';
 
+const _packagesFileName = '.packages';
+
 /// Extend to get a command with the arguments common to all build_runner
 /// commands.
 abstract class BuildRunnerCommandBase extends Command<int> {
@@ -30,6 +32,8 @@ abstract class BuildRunnerCommandBase extends Command<int> {
   Future<int> runCore(String command) async {
     await checkPubspecLock();
 
+    var buildRunnerScript = await _buildRunnerScript();
+
     final arguments = [command]..addAll(argResults.arguments);
 
     var exitCode = 0;
@@ -49,8 +53,7 @@ abstract class BuildRunnerCommandBase extends Command<int> {
       stderr.writeln(new Trace.parse(trace).terse);
       if (exitCode == 0) exitCode = 1;
     });
-    await Isolate.spawnUri(
-        await _buildRunnerScript(), arguments, messagePort.sendPort,
+    await Isolate.spawnUri(buildRunnerScript, arguments, messagePort.sendPort,
         onExit: exitPort.sendPort,
         onError: errorPort.sendPort,
         automaticPackageResolution: true);
@@ -74,6 +77,13 @@ abstract class BuildRunnerCommandBase extends Command<int> {
 }
 
 Future<Uri> _buildRunnerScript() async {
+  var packagesFile = new File(_packagesFileName);
+  if (!packagesFile.existsSync()) {
+    throw new FileSystemException(
+        'A `$_packagesFileName` file does not exist in the target directory.',
+        packagesFile.absolute.path);
+  }
+
   var dataUri = new Uri.dataFromString(_bootstrapScript);
 
   var messagePort = new ReceivePort();
@@ -84,7 +94,7 @@ Future<Uri> _buildRunnerScript() async {
       onExit: exitPort.sendPort,
       onError: errorPort.sendPort,
       errorsAreFatal: true,
-      packageConfig: new Uri.file('.packages'));
+      packageConfig: new Uri.file(_packagesFileName));
 
   var allErrorsFuture = errorPort.forEach((error) {
     var errorList = error as List;
@@ -106,7 +116,7 @@ Future<Uri> _buildRunnerScript() async {
 
   var messages = items[0] as List;
   if (messages.isEmpty) {
-    throw new StateError('An error occurred while running booting.');
+    throw new StateError('An error occurred while bootstrapping.');
   }
 
   assert(messages.length == 1);

webdev/test/integration_test.dart

@@ -97,4 +97,34 @@ packages:
         contains('A dependency on `build_web_compilers` was not found.'));
     await process.shouldExit(78);
   });
+
+  test('should fail gracefully if there is no .packages file', () async {
+    await d.file('pubspec.lock', '''
+# Copy-pasted from a valid run
+packages:
+  build_runner:
+    dependency: "direct main"
+    description:
+      name: build_runner
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "0.8.0"
+  build_web_compilers:
+    dependency: "direct main"
+    description:
+      name: build_web_compilers
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "0.3.4+2"
+''').create();
+
+    var process = await TestProcess.start('dart', [_webdevBin, 'build'],
+        workingDirectory: d.sandbox);
+    var output = (await process.stdoutStream().join('\n')).trim();
+
+    expect(output, contains('Could not run in the current directory.'));
+    expect(output,
+        contains('A `.packages` file does not exist in the target directory.'));
+    await process.shouldExit(78);
+  });
 }