Reapply conversion of most core interfaces to abstract classes.

lib/core/stopwatch.dart

Index: lib/core/stopwatch.dart
diff --git a/lib/core/stopwatch.dart b/lib/core/stopwatch.dart
index 6222d672abd56164ca6b7bf1a169c30d2df5b427..e43f52d4bac7ced5274815a6b56b7d3910e81456 100644
--- a/lib/core/stopwatch.dart
+++ b/lib/core/stopwatch.dart
@@ -5,8 +5,7 @@
 /**
  * A simple [Stopwatch] interface to measure elapsed time.
  */
-interface Stopwatch default StopwatchImplementation {
-
+abstract class Stopwatch {
   /**
    * Creates a [Stopwatch] in stopped state with a zero elapsed count.
    *
@@ -15,7 +14,7 @@ interface Stopwatch default StopwatchImplementation {
    *
    *     Stopwatch stopwatch = new Stopwatch()..start();
    */
-  Stopwatch();
+  factory Stopwatch() => new StopwatchImplementation();
 
   /**
    * Starts the [Stopwatch]. The [elapsed] count is increasing monotonically.