Fix some warnings in the core library identified by the static analyzer.

runtime/lib/mirrors_impl.dart

Index: runtime/lib/mirrors_impl.dart
diff --git a/runtime/lib/mirrors_impl.dart b/runtime/lib/mirrors_impl.dart
index 90743a5e78ba7f51322fa619b7f64e64eab3d440..a743796045a32104d64864594ccf17b9e42f9bc2 100644
--- a/runtime/lib/mirrors_impl.dart
+++ b/runtime/lib/mirrors_impl.dart
@@ -50,8 +50,8 @@ abstract class _LocalObjectMirrorImpl extends _LocalVMObjectMirrorImpl
   _LocalObjectMirrorImpl(ref) : super(ref) {}
 
   Future<InstanceMirror> invoke(String memberName,
-                                List<Object> positionalArguments,
-                                [Map<String,Object> namedArguments]) {
+                                List positionalArguments,
+                                [Map<String,Dynamic> namedArguments]) {
     if (namedArguments !== null) {
       throw new NotImplementedException('named arguments not implemented');
     }