Silence more new analyzer warnings (#6874)

See #6861

packages/flutter_test/lib/src/binding.dart

@@ -112,7 +112,7 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
   void initInstances() {
     timeDilation = 1.0; // just in case the developer has artificially changed it for development
     http.Client.clientOverride = () {
-      return new http.MockClient((http.Request request){
+      return new http.MockClient((http.BaseRequest request) {
         return new Future<http.Response>.value(
           new http.Response("Mocked: Unavailable.", 404, request: request)
         );

packages/flutter_test/lib/src/controller.dart

@@ -74,7 +74,11 @@ class WidgetController {
   /// * Use [firstWidget] if you expect to match several but only want the first.
   Iterable<Widget/*=T*/> widgetList/*<T extends Widget>*/(Finder finder) {
     TestAsyncUtils.guardSync();
-    return finder.evaluate().map/*<T>*/((Element element) => element.widget);
+    return finder.evaluate().map/*<T>*/((Element element) {
+      // TODO(ianh): simplify once the VM can infer the return type
+      dynamic/*=T*/ result = element.widget;
+      return result;
+    });
   }
 
 
@@ -219,7 +223,11 @@ class WidgetController {
   /// * Use [firstRenderObject] if you expect to match several but only want the first.
   Iterable<RenderObject/*=T*/> renderObjectList/*<T extends RenderObject>*/(Finder finder) {
     TestAsyncUtils.guardSync();
-    return finder.evaluate().map/*<T>*/((Element element) => element.renderObject);
+    return finder.evaluate().map/*<T>*/((Element element) {
+      // TODO(ianh): simplify once the VM can infer the return type
+      dynamic/*=T*/ result = element.renderObject;
+      return result;
+    });
   }