Add stack chain support to pub.

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

Index: sdk/lib/_internal/pub/lib/src/entrypoint.dart
diff --git a/sdk/lib/_internal/pub/lib/src/entrypoint.dart b/sdk/lib/_internal/pub/lib/src/entrypoint.dart
index baeb3f3616f7e27ec6a3d4bf9f0577b2aabfd276..3882f2e5714535acd1e8ec06453d2264391f7d3a 100644
--- a/sdk/lib/_internal/pub/lib/src/entrypoint.dart
+++ b/sdk/lib/_internal/pub/lib/src/entrypoint.dart
@@ -80,7 +80,7 @@ class Entrypoint {
     var packageDir = path.join(packagesDir, id.name);
     var source;
 
-    var future = new Future.sync(() {
+    var future = syncFuture(() {
       ensureDir(path.dirname(packageDir));
 
       if (entryExists(packageDir)) {
@@ -126,7 +126,7 @@ class Entrypoint {
       bool upgradeAll: false}) {
     var numChanged = 0;
 
-    return new Future.sync(() {
+    return syncFuture(() {
       return resolveVersions(cache.sources, root, lockFile: loadLockFile(),
           useLatest: useLatest, upgradeAll: upgradeAll);
     }).then((result) {
@@ -212,7 +212,7 @@ class Entrypoint {
   /// Gets dependencies if the lockfile is out of date with respect to the
   /// pubspec.
   Future ensureLockFileIsUpToDate() {
-    return new Future.sync(() {
+    return syncFuture(() {
       var lockFile = loadLockFile();
 
       // If we don't have a current lock file, we definitely need to install.