Adding some asset-related utilities to code_transformers

pkg/code_transformers/lib/src/entry_point.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+import 'dart:async';
+
+import 'package:analyzer/analyzer.dart' as analyzer;
+import 'package:analyzer/src/generated/ast.dart';
+import 'package:barback/barback.dart';
+
+/// Checks to see if the provided Asset is a Dart entry point.
+///
+/// Assets are considered entry points if they are Dart files located in
+/// web/, test/, benchmark/ or example/ and have a main() function.
+///
+/// Because this only analyzes the primary asset this may return true for files
+/// which are not dart entries if the file does not have a main() but does have
+/// parts or exports.
+Future<bool> isPossibleDartEntry(Asset asset) {
+  if (asset.id.extension != '.dart') return new Future.value(false);
+
+  if (!['benchmark', 'example', 'test', 'web']
+      .any((dir) => asset.id.path.startsWith("$dir/"))) {
+    return new Future.value(false);
+  }
+  return asset.readAsString().then((contents) {
+    return _couldBeEntrypoint(analyzer.parseCompilationUnit(contents));
+  });
+}
+
+bool _couldBeEntrypoint(CompilationUnit compilationUnit) {
+  // Allow two or fewer arguments so that entrypoints intended for use with
+  // [spawnUri] get counted.
+  var hasMain = compilationUnit.declarations.any((node) =>
+      node is FunctionDeclaration &&
+      node.name.name == "main" &&
+      node.functionExpression.parameters.parameters.length <= 2);
+
+  if (hasMain) return true;
+
+  // If it has an export or a part, assume the worst- that the main could be
+  // in there.
+  // We avoid loading those since this can be run from isPrimaryAsset calls
+  // where we do not have access to other sources.
+  return compilationUnit.directives.any((node) =>
+      node is ExportDirective || node is PartDirective);
+}