Refactor Source and SourceRegistry.

lib/src/package_graph.dart

Index: lib/src/package_graph.dart
diff --git a/lib/src/package_graph.dart b/lib/src/package_graph.dart
index ae603ed2e388ed3e26bb264df2f8ff38ba0252dd..f6b4a9317811de7fb20dca417a4c4f0bd3c55ef0 100644
--- a/lib/src/package_graph.dart
+++ b/lib/src/package_graph.dart
@@ -48,7 +48,7 @@ class PackageGraph {
       if (id.name == entrypoint.root.name) return entrypoint.root;
 
       return new Package(result.pubspecs[id.name],
-          entrypoint.cache.sources[id.source].getDirectory(id));
+          entrypoint.cache.source(id.source).getDirectory(id));
     });
 
     return new PackageGraph(entrypoint, result.lockFile, packages);
@@ -125,8 +125,7 @@ class PackageGraph {
     var id = lockFile.packages[package];
     if (id == null) return true;
 
-    var source = entrypoint.cache.sources[id.source];
-    if (source is! CachedSource) return true;
+    if (entrypoint.cache.source(id.source) is! CachedSource) return true;
 
     return transitiveDependencies(package).any((dep) {
       var depId = lockFile.packages[dep.name];
@@ -135,7 +134,7 @@ class PackageGraph {
       // mutable.
       if (depId == null) return true;
 
-      return entrypoint.cache.sources[depId.source] is! CachedSource;
+      return entrypoint.cache.source(depId.source) is! CachedSource;
     });
   }
 
@@ -150,10 +149,7 @@ class PackageGraph {
   bool isPackageStatic(String package) {
     var id = lockFile.packages[package];
     if (id == null) return false;
-
-    var source = entrypoint.cache.sources[id.source];
-    if (source is! CachedSource) return false;
-
+    if (entrypoint.cache.source(id.source) is! CachedSource) return false;
     return packages[package].pubspec.transformers.isEmpty;
   }
 }