Fix all strong-mode warnings.

lib/src/graph/package_graph.dart

Index: lib/src/graph/package_graph.dart
diff --git a/lib/src/graph/package_graph.dart b/lib/src/graph/package_graph.dart
index 75730819e636aa6ce406ff0870bab9d19e86ee29..1c2b534e08113ba55e616cbb24bc33e28d6e43ec 100644
--- a/lib/src/graph/package_graph.dart
+++ b/lib/src/graph/package_graph.dart
@@ -127,7 +127,7 @@ class PackageGraph {
   ///
   /// If the asset cannot be found, returns null.
   Future<AssetNode> getAssetNode(AssetId id) {
-    return _inErrorZone(() {
+    return _inErrorZone/*<Future<AssetNode>>*/(() {
       var cascade = _cascades[id.package];
       if (cascade != null) return cascade.getAssetNode(id);
       return new Future.value(null);
@@ -158,7 +158,7 @@ class PackageGraph {
 
     if (_status != NodeStatus.IDLE) {
       // A build is still ongoing, so wait for it to complete and try again.
-      return results.first.then((_) => getAllAssets());
+      return results.first.then/*<Future<AssetSet>>*/((_) => getAllAssets());
     }
 
     // If an unexpected error occurred, complete with that.
@@ -178,7 +178,7 @@ class PackageGraph {
             (cascade) => cascade.availableOutputs))
           .then((assetSets) {
       var assets = unionAll(assetSets.map((assetSet) => assetSet.toSet()));
-      return new Future.value(new AssetSet.from(assets));
+      return new AssetSet.from(assets);
     });
   }
 
@@ -274,10 +274,11 @@ class PackageGraph {
   /// [Future]. If it throws a [BarbackException], that exception will be piped
   /// to the returned [Future] as well. Any other exceptions will be piped to
   /// [results].
-  Future _inErrorZone(body()) {
-    var completer = new Completer.sync();
+  Future/*<T>*/ _inErrorZone/*<T>*/(/*=T*/ body()) {
+    var completer = new Completer/*<T>*/.sync();
     runZoned(() {
-      syncFuture(body).then(completer.complete).catchError((error, stackTrace) {
+      new Future.sync(body).then(completer.complete)
+          .catchError((error, stackTrace) {
         if (error is! BarbackException) throw error;
         completer.completeError(error, stackTrace);
       });