better support rebuilds after package graph updates

e2e_example/.gitignore

@@ -1 +1,2 @@
 **/*.txt.copy
+web/_packages

e2e_example/lib/packages_dir_builder.dart

@@ -0,0 +1,42 @@
+// Copyright (c) 2016, 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:path/path.dart' as p;
+
+import 'package:build/build.dart';
+import 'package:build/src/generate/input_set.dart';
+import 'package:build/src/generate/phase.dart';
+
+// Makes copies of things!
+class PackagesDirBuilder extends Builder {
+  final String outputDir;
+  final String outputPackage;
+
+  PackagesDirBuilder(this.outputPackage, this.outputDir);
+
+  @override
+  Future build(BuildStep buildStep) async {
+    var input = buildStep.input;
+    var copy = new Asset(_copiedAssetId(input.id), input.stringContents);
+    await buildStep.writeAsString(copy);
+  }
+
+  @override
+  List<AssetId> declareOutputs(AssetId inputId) => [_copiedAssetId(inputId)];
+
+  static void addPhases(PhaseGroup group, PackageGraph graph) {
+    var builder = new PackagesDirBuilder(graph.root.name, 'web');
+    var phase = group.newPhase();
+    for (var package in graph.allPackages.values) {
+      phase.addAction(builder, new InputSet(package.name, ['lib/**']));
+    }
+  }
+
+  AssetId _copiedAssetId(AssetId inputId) => new AssetId(
+      outputPackage,
+      // Creating a folder actually called `packages` breaks things.
+      p.join(outputDir, '_packages', inputId.package,
+          inputId.path.replaceFirst('lib/', '')));
+}

e2e_example/tool/watch.dart

@@ -6,16 +6,20 @@ import 'dart:async';
 import 'package:build/build.dart';
 
 import 'package:e2e_example/copy_builder.dart';
+import 'package:e2e_example/packages_dir_builder.dart';
 
 Future main() async {
-  /// Builds a full package dependency graph for the current package.
+  // Builds a full package dependency graph for the current package.
   var graph = new PackageGraph.forThisPackage();
   var phases = new PhaseGroup();
 
-  /// Give [Builder]s access to a [PackageGraph] so they can choose which
-  /// packages to run on. This simplifies user code a lot, and helps to mitigate
-  /// the transitive deps issue.
+  // Give [Builder]s access to a [PackageGraph] so they can choose which
+  // packages to run on. This simplifies user code a lot, and helps to mitigate
+  // the transitive deps issue.
   CopyBuilder.addPhases(phases, graph);
 
+  // Adds all the phases necessary to copy all files into a fake packages dir!
+  PackagesDirBuilder.addPhases(phases, graph);
+
   watch(phases);
 }

lib/src/asset/file_based.dart

@@ -47,13 +47,17 @@ class FileBasedAssetReader implements AssetReader {
       var packageNode = packageGraph[inputSet.package];
       if (packageNode == null) {
         throw new ArgumentError(
-            "Could not find inputSet.package '${inputSet.package}'.");
+            "Could not find package '${inputSet.package}' which was listed as "
+            "an input. Please ensure you have that package in your deps, or "
+            "remove it from your input sets.");
       }
       var packagePath = packageNode.location.toFilePath();
       for (var glob in inputSet.globs) {
         var fileStream = glob.list(followLinks: false, root: packagePath).where(
             (e) => e is File && !ignoredDirs.contains(path.split(e.path)[1]));
         await for (var entity in fileStream) {
+          // TODO(jakemac): Where do these files come from???
+          if (path.basename(entity.path).startsWith('._')) continue;
           var id = _fileToAssetId(entity, packageNode);
           if (!seenAssets.add(id)) continue;
           yield id;

lib/src/generate/build_impl.dart

@@ -256,7 +256,12 @@ class BuildImpl {
             node is! GeneratedAssetNode ||
             (node as GeneratedAssetNode).wasOutput)
         .map((node) async {
-      var exists = await _reader.hasInput(node.id);
+      bool exists;
+      try {
+        exists = await _reader.hasInput(node.id);
+      } on PackageNotFoundException catch(_) {
+        exists = false;
+      }
       if (!exists) {
         updates[node.id] = ChangeType.REMOVE;
         return;