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..3c7df3e357225c0807bbe02d650497056979407d 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 +60,28 @@ 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) {

[nweiz, 2013/01/31 22:38:53]

I don't like that this returns a Package whereas other describe() methods return
Pubspecs. It looks to me like you could make walkDependencies work with Pubspecs
rather than Packages, so I'd do that.

[Bob Nystrom, 2013/02/01 16:53:29]

I had that originally, but Pubspec doesn't know how to find and read the file.
It's constructor just takes the contents of a pubspec file. So I had code here
that would locate the pubspec file, read it, and then create the pubspec.

That's exactly what Package's constructor does. So it seemed like Package was a
better object to use.

[nweiz, 2013/02/01 22:24:54]

I think it's fine to use Package internally to load the pubspec, but I don't
think it makes sense to return it. It's not useful for the known use-case, and
it's inconsistent with other methods called "describe".

If you really feel like this should return a package, at least rename it load()
or something like that.

[Bob Nystrom, 2013/02/02 00:02:31]

Done.

+    Future<Package> getUncached() {
+      // Not cached, so get it from the source.
+      return id.source.describe(id).then((pubspec) {

[nweiz, 2013/01/31 22:38:53]

"id.source.describe(id)" -> "id.describe()"

[Bob Nystrom, 2013/02/01 16:53:29]

Done.

+        return new Package.inMemory(pubspec);
+      });
+    }
+
+    // Try to get it from the system cache first.
+    if (id.source.shouldCache) {
+      return id.source.systemCacheDirectory(id).then((packageDir) {

[nweiz, 2013/01/31 22:38:53]

This would be a little cleaner with an "id.systemCacheDirectory" getter.

[Bob Nystrom, 2013/02/01 16:53:29]

Done.

+        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.
   ///