Change package scheduled_test to throw TestFailures rather than strings.

pkg/scheduled_test/lib/src/descriptor/file_descriptor.dart

Index: pkg/scheduled_test/lib/src/descriptor/file_descriptor.dart
diff --git a/pkg/scheduled_test/lib/src/descriptor/file_descriptor.dart b/pkg/scheduled_test/lib/src/descriptor/file_descriptor.dart
index eca863ae772dadbe669266d36fe3715e57f422b3..9111b3dd656824a6a479bfd1610d70727fe21d7b 100644
--- a/pkg/scheduled_test/lib/src/descriptor/file_descriptor.dart
+++ b/pkg/scheduled_test/lib/src/descriptor/file_descriptor.dart
@@ -59,7 +59,7 @@ abstract class FileDescriptor extends Descriptor implements ReadableDescriptor {
     if (parent == null) parent = defaultRoot;
     var fullPath = path.join(parent, name);
     if (!new File(fullPath).existsSync()) {
-      throw "File not found: '$fullPath'.";
+      fail("File not found: '$fullPath'.");
     }
 
     return new File(fullPath).readAsBytes().then(_validateNow);
@@ -84,7 +84,7 @@ class _BinaryFileDescriptor extends FileDescriptor {
   Future _validateNow(List<int> actualContents) {
     if (orderedIterableEquals(contents, actualContents)) return null;
     // TODO(nweiz): show a hex dump here if the data is small enough.
-    throw "File '$name' didn't contain the expected binary data.";
+    fail("File '$name' didn't contain the expected binary data.");
   }
 }