Add a toString to Simulation (#7364)

Also, make hasOneLineDescription more discerning.
Also, add a test for hasOneLineDescription.
Also, add a test for GravitySimulation, to test the toString.

packages/flutter/lib/src/physics/simulation.dart

@@ -47,4 +47,7 @@ abstract class Simulation {
   /// asymptote itself could not be seen, it would be pointless to continue. The
   /// tolerance defines how to determine if the difference could not be seen.
   Tolerance tolerance = Tolerance.defaultTolerance;
+
+  @override
+  String toString() => '$runtimeType';
 }

packages/flutter/test/physics/gravity_simulation_test.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/physics.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('gravity simulation', () {
+    expect(new GravitySimulation(9.81, 10.0, 0.0, 0.0), hasOneLineDescription);
+    expect(new GravitySimulation(9.81, 10.0, 0.0, 0.0).x(10.0), moreOrLessEquals(50.0 * 9.81 + 10.0));
+  });
+}

packages/flutter/test/physics/near_equal_test.dart

-// Copyright (c) 2015 The Chromium Authors. All rights reserved.
+// Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 

packages/flutter/test/physics/newton_test.dart

-// Copyright (c) 2015 The Chromium Authors. All rights reserved.
+// Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 

packages/flutter_test/lib/src/matchers.dart

@@ -62,8 +62,8 @@ const Matcher isNotInCard = const _IsNotInCard();
 /// Asserts that an object's toString() is a plausible one-line description.
 ///
 /// Specifically, this matcher checks that the string does not contains newline
-/// characters, and does not have leading or trailing whitespace, and is not
-/// empty.
+/// characters, and does not have leading or trailing whitespace, is not
+/// empty, and does not contain the default `Instance of ...` string.
 const Matcher hasOneLineDescription = const _HasOneLineDescription();
 
 /// Asserts that two [double]s are equal, within some tolerated error.
@@ -242,6 +242,7 @@ class _HasOneLineDescription extends Matcher {
     String description = object.toString();
     return description.isNotEmpty
         && !description.contains('\n')
+        && !description.contains('Instance of ')
         && description.trim() == description;
   }
 

packages/flutter_test/test/matchers_test.dart

@@ -5,6 +5,14 @@
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
+  test('hasOneLineDescription', () {
+    expect('Hello', hasOneLineDescription);
+    expect('Hello\nHello', isNot(hasOneLineDescription));
+    expect(' Hello', isNot(hasOneLineDescription));
+    expect('Hello ', isNot(hasOneLineDescription));
+    expect(new Object(), isNot(hasOneLineDescription));
+  });
+
   test('moreOrLessEquals', () {
     expect(0.0, moreOrLessEquals(1e-11));
     expect(1e-11, moreOrLessEquals(0.0));