Print the stack traces for Future expectations that raise exceptions.

lib/unittest/core_matchers.dart

Index: lib/unittest/core_matchers.dart
diff --git a/lib/unittest/core_matchers.dart b/lib/unittest/core_matchers.dart
index c0b1d55040ff604edd2d8d3c01d10d88a646c205..d24518d4769c9f973cdc7ac3d624cafc17101f61 100644
--- a/lib/unittest/core_matchers.dart
+++ b/lib/unittest/core_matchers.dart
@@ -280,7 +280,13 @@ class _Throws extends BaseMatcher {
           expect(false, reason:
               "Expected future to fail, but succeeded with '${future.value}'.");
         } else if (_matcher != null) {
-          expect(future.exception, _matcher);
+          var reason;
+          if (future.stackTrace != null) {
+            var stackTrace = future.stackTrace.toString();
+            stackTrace = "  ${stackTrace.replaceAll("\n", "\n  ")}";
+            reason = "Actual exception trace:\n$stackTrace";
+          }
+          expect(future.exception, _matcher, reason: reason);
         }
       }));