Gracefully handle pubspecs with dependencies using unknown sources.

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

Index: sdk/lib/_internal/pub/lib/src/hosted_source.dart
diff --git a/sdk/lib/_internal/pub/lib/src/hosted_source.dart b/sdk/lib/_internal/pub/lib/src/hosted_source.dart
index 17b18399cb209e16c3066aa69418202f37d0c0f0..62503fe1274f6b2d3a1c7f1c28837f512a96310c 100644
--- a/sdk/lib/_internal/pub/lib/src/hosted_source.dart
+++ b/sdk/lib/_internal/pub/lib/src/hosted_source.dart
@@ -48,7 +48,7 @@ class HostedSource extends Source {
 
   /// Downloads and parses the pubspec for a specific version of a package that
   /// is available from the site.
-  Future<Pubspec> describe(PackageId id) {
+  Future<Pubspec> describeUncached(PackageId id) {
     // Request it from the server.
     var url = _makeVersionUrl(id, (server, package, version) =>
         "$server/packages/$package/versions/$version.yaml");
@@ -191,9 +191,9 @@ class OfflineHostedSource extends HostedSource {
     throw new UnsupportedError("Cannot install packages when offline.");
   }
 
-  Future<Pubspec> describe(PackageId id) {
+  Future<Pubspec> describeUncached(PackageId id) {
     // [getVersions()] will only return packages that are already cached.
-    // SystemCache should only call [describe()] on a package after it has
+    // Source should only call [describeUncached()] on a package after it has
     // failed to find it in the cache, so this code should not be reached.
     throw new UnsupportedError("Cannot describe packages when offline.");
   }