Validate packages against their SDK constraints.

utils/pub/package.dart

Index: utils/pub/package.dart
diff --git a/utils/pub/package.dart b/utils/pub/package.dart
index 15fcb97b65874279b40ab650285091dcb6ba299e..72c5c0e19c77476b616d4bcb8a48ed9034e95100 100644
--- a/utils/pub/package.dart
+++ b/utils/pub/package.dart
@@ -58,7 +58,7 @@ class Package {
 
   /// The ids of the packages that this package depends on. This is what is
   /// specified in the pubspec when this package depends on another.
-  Collection<PackageRef> get dependencies => pubspec.dependencies;
+  List<PackageRef> get dependencies => pubspec.dependencies;
 
   /// Returns the path to the README file at the root of the entrypoint, or null
   /// if no README file is found. If multiple READMEs are found, this uses the
@@ -124,6 +124,10 @@ class PackageId implements Comparable {
 
   int get hashCode => name.hashCode ^ source.hashCode ^ version.hashCode;
 
+  /// Gets the directory where this package is or would be found in the
+  /// [SystemCache].
+  Future<String> get systemCacheDirectory => source.systemCacheDirectory(this);
+
   bool operator ==(other) {
     if (other is! PackageId) return false;
     // TODO(rnystrom): We're assuming here the name/version/source tuple is