First pass at build dependency graph for barback.

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

Index: sdk/lib/_internal/pub/lib/src/system_cache.dart
diff --git a/sdk/lib/_internal/pub/lib/src/system_cache.dart b/sdk/lib/_internal/pub/lib/src/system_cache.dart
index 63de5dc59899e0acff87082e84ad415d82eebbf1..adca55591e3647c02f3435cb1d0321abedfe55b4 100644
--- a/sdk/lib/_internal/pub/lib/src/system_cache.dart
+++ b/sdk/lib/_internal/pub/lib/src/system_cache.dart
@@ -41,15 +41,25 @@ class SystemCache {
 
   /// Creates a new package cache which is backed by the given directory on the
   /// user's file system.
-  SystemCache(this.rootDir)
-  : _pendingInstalls = new Map<PackageId, Future<Package>>(),
-    sources = new SourceRegistry();
+  factory SystemCache() {
+    var cacheDir;
+    if (Platform.environment.containsKey('PUB_CACHE')) {
+      cacheDir = Platform.environment['PUB_CACHE'];
+    } else if (Platform.operatingSystem == 'windows') {
+      var appData = Platform.environment['APPDATA'];
+      cacheDir = path.join(appData, 'Pub', 'Cache');
+    } else {
+      cacheDir = '${Platform.environment['HOME']}/.pub-cache';
+    }
+
+    return new SystemCache._(cacheDir);
+  }
 
   /// Creates a system cache and registers the standard set of sources. If
   /// [isOffline] is `true`, then the offline hosted source will be used.
   /// Defaults to `false`.
-  factory SystemCache.withSources(String rootDir, {bool isOffline: false}) {
-    var cache = new SystemCache(rootDir);
+  factory SystemCache.withSources({bool isOffline: false}) {
+    var cache = new SystemCache();
     cache.register(new GitSource());
 
     if (isOffline) {
@@ -63,6 +73,10 @@ class SystemCache {
     return cache;
   }
 
+  SystemCache._(this.rootDir)
+      : _pendingInstalls = new Map<PackageId, Future<Package>>(),
+        sources = new SourceRegistry();
+
   /// Registers a new source. This source must not have the same name as a
   /// source that's already been registered.
   void register(Source source) {