Validate packages against their SDK constraints.

utils/pub/source.dart

Index: utils/pub/source.dart
diff --git a/utils/pub/source.dart b/utils/pub/source.dart
index 211f0dbad4dc342350d9714ee83e5d18d38858f0..d086374f3bffcde40dd21c2b202bcdd582bfbc80 100644
--- a/utils/pub/source.dart
+++ b/utils/pub/source.dart
@@ -104,13 +104,14 @@ abstract class Source {
   ///
   /// By default, this uses [systemCacheDirectory] and [install].
   Future<Package> installToSystemCache(PackageId id) {
-    var path = systemCacheDirectory(id);
-    return exists(path).then((exists) {
-      if (exists) return new Future<bool>.immediate(true);
-      return ensureDir(dirname(path)).then((_) => install(id, path));
-    }).then((found) {
-      if (!found) throw 'Package $id not found.';
-      return Package.load(id.name, path, systemCache.sources);
+    return systemCacheDirectory(id).then((path) {
+      return exists(path).then((exists) {
+        if (exists) return new Future<bool>.immediate(true);
+        return ensureDir(dirname(path)).then((_) => install(id, path));
+      }).then((found) {
+        if (!found) throw 'Package $id not found.';
+        return Package.load(id.name, path, systemCache.sources);
+      });
     });
   }
 
@@ -118,11 +119,10 @@ abstract class Source {
   /// [id] should be installed to. This should return a path to a subdirectory
   /// of [systemCacheRoot].
   ///
-  /// This doesn't need to be implemented if [shouldCache] is false, or if
-  /// [installToSystemCache] is implemented.
-  String systemCacheDirectory(PackageId id) {
-    throw 'Source.systemCacheDirectory must be implemented if shouldCache is '
-        'true and installToSystemCache is not implemented.';
+  /// This doesn't need to be implemented if [shouldCache] is false.
+  Future<String> systemCacheDirectory(PackageId id) {
+    return new Future.immediateError(
+        "systemCacheDirectory() must be implemented if shouldCache is true.");
   }
 
   /// When a [Pubspec] or [LockFile] is parsed, it reads in the description for