Precompile immutable dependencies on "pub get".

sdk/lib/_internal/pub/lib/src/package_graph.dart

 // Copyright (c) 2013, 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.
 
 library pub.package_graph;
 
 import 'barback/transformer_cache.dart';
 import 'entrypoint.dart';
 import 'lock_file.dart';
 import 'package.dart';
+import 'source/cached.dart';
+import 'source/hosted.dart';
 import 'utils.dart';
 
 /// A holistic view of the entire transitive dependency graph for an entrypoint.
 ///
 /// A package graph can be loaded using [Entrypoint.loadPackageGraph].
 class PackageGraph {
   /// The entrypoint.
   final Entrypoint entrypoint;
 
   /// The entrypoint's lockfile.
@@ skipping 38 matching lines @@
 
     if (_transitiveDependencies == null) {
       var closure = transitiveClosure(mapMap(packages,
           value: (_, package) => package.dependencies.map((dep) => dep.name)));
       _transitiveDependencies = mapMap(closure,
           value: (_, names) => names.map((name) => packages[name]).toSet());
     }
 
     return _transitiveDependencies[package];
   }
+
+  /// Returns whether [package] is mutable.
+  ///
+  /// A package is considered to be mutable if it or any of its dependencies
+  /// don't come from a cached source, since the user can change its contents
+  /// without modifying the pub cache. Information generated from mutable
+  /// packages is generally not safe to cache, since it may change frequently.
+  bool isPackageMutable(String package) {
+    var id = lockFile.packages[package];
+    if (id == null) return true;
+
+    var source = entrypoint.cache.sources[id.source];
+    if (source is! CachedSource) return true;
+
+    return transitiveDependencies(package).any((dep) {
+      var depId = lockFile.packages[dep.name];
+
+      // The entrypoint package doesn't have a lockfile entry. It's always
+      // mutable.
+      if (depId == null) return true;
+
+      return entrypoint.cache.sources[depId.source] is! CachedSource;
+    });
+  }
 }