Fix Dart 2 runtime issue; type function better (#66)

Fix Dart 2 runtime issue; type function better

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 309dfb4..1ba2046 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,7 +2,6 @@
 
 dart:
   - dev
-  - stable
 
 dart_task:
   - test: --platform vm

lib/src/async_benchmark_base.dart

diff --git a/lib/src/async_benchmark_base.dart b/lib/src/async_benchmark_base.dart
index 423ff64..3098b5e 100644
--- a/lib/src/async_benchmark_base.dart
+++ b/lib/src/async_benchmark_base.dart
@@ -37,7 +37,7 @@
 
   // Measures the score for this benchmark by executing it repeatedly until
   // time minimum has been reached.
-  static Future<double> measureFor(Function f, int minimumMillis) {
+  static Future<double> measureFor(Future Function() f, int minimumMillis) {
     int minimumMicros = minimumMillis * 1000;
     int iter = 0;
     Stopwatch watch = new Stopwatch();

test/messages_test.dart

diff --git a/test/messages_test.dart b/test/messages_test.dart
index 526263d..15bc4bb 100644
--- a/test/messages_test.dart
+++ b/test/messages_test.dart
@@ -97,8 +97,8 @@
         expect(table.entries[_id('hi', 11)].length, 2);
         expect(table.entries[_id('hi', 13)].length, 1);
 
-        var table2 =
-            new LogEntryTable.fromJson(toJson(table) as Map<String, Iterable>);
+        var table2 = new LogEntryTable.fromJson(
+            (toJson(table) as Map).cast<String, Iterable>());
         expect(table2.entries.length, 2);
         expect(table2.entries[_id('hi', 11)].length, 2);
         expect(table2.entries[_id('hi', 13)].length, 1);