Validate packages against their SDK constraints.

utils/pub/system_cache.dart

Index: utils/pub/system_cache.dart
diff --git a/utils/pub/system_cache.dart b/utils/pub/system_cache.dart
index 1e26554cd72a177088c1a83779d6940b07e50b1d..634dc173257b0e9742a549f86e6903fae9b855be 100644
--- a/utils/pub/system_cache.dart
+++ b/utils/pub/system_cache.dart
@@ -13,6 +13,7 @@ import 'io.dart';
 import 'io.dart' as io show createTempDir;
 import 'log.dart' as log;
 import 'package.dart';
+import 'pubspec.dart';
 import 'sdk_source.dart';
 import 'source.dart';
 import 'source_registry.dart';
@@ -59,6 +60,26 @@ class SystemCache {
     sources.register(source);
   }
 
+  /// Gets the package identified by [id]. If the package is already cached,
+  /// reads it from the cache. Otherwise, requests it from the source.
+  Future<Package> describe(PackageId id) {
+    Future<Package> getUncached() {
+      // Not cached, so get it from the source.
+      return id.describe().then((pubspec) => new Package.inMemory(pubspec));
+    }
+
+    // Try to get it from the system cache first.
+    if (id.source.shouldCache) {
+      return id.systemCacheDirectory.then((packageDir) {
+        if (!dirExistsSync(packageDir)) return getUncached();
+        return Package.load(id.name, packageDir, sources);
+      });
+    }
+
+    // Not cached, so get it from the source.
+    return getUncached();
+  }
+
   /// Ensures that the package identified by [id] is installed to the cache,
   /// loads it, and returns it.
   ///