Fix all strong-mode warnings.

test/transformer/one_to_many.dart

Index: test/transformer/one_to_many.dart
diff --git a/test/transformer/one_to_many.dart b/test/transformer/one_to_many.dart
index 0076680a75915585dc23241de796c95b250c0460..05bea4dd2dcef42760787a6f58a7e4538d5ef5eb 100644
--- a/test/transformer/one_to_many.dart
+++ b/test/transformer/one_to_many.dart
@@ -23,15 +23,12 @@ class OneToManyTransformer extends MockTransformer {
 
   bool doIsPrimary(AssetId id) => id.extension == ".$extension";
 
-  Future doApply(Transform transform) {
-    return getPrimary(transform)
-        .then((input) => input.readAsString())
-        .then((lines) {
-      for (var line in lines.split(",")) {
-        var id = new AssetId(transform.primaryInput.id.package, line);
-        transform.addOutput(new Asset.fromString(id, "spread $extension"));
-      }
-    });
+  Future doApply(Transform transform) async {
+    var lines = await (await getPrimary(transform)).readAsString();
+    for (var line in lines.split(",")) {
+      var id = new AssetId(transform.primaryInput.id.package, line);
+      transform.addOutput(new Asset.fromString(id, "spread $extension"));
+    }
   }
 
   String toString() => "1->many $extension";