moreOrLessEquals matcher for floating point numbers (#7358)

```dart
expect(x, moreOrLessEquals(0.0));
```

...for those cases where `x` might be 1e-11 or whatever.

Also, be more resilient when dumping the tree from inside the test framework.
Also, add an assert that helped me debug something the other day.

packages/flutter_test/lib/src/all_elements.dart

@@ -49,6 +49,7 @@ class _DepthFirstChildIterator implements Iterator<Element> {
   }
 
   static Iterable<Element> _reverseChildrenOf(Element element, bool skipOffstage) {
+    assert(element != null);
     final List<Element> children = <Element>[];
     if (skipOffstage) {
       element.visitChildrenForSemantics(children.add);

packages/flutter_test/lib/src/binding.dart

@@ -334,7 +334,12 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
         // _this_ zone, the test framework would find this zone was the current
         // zone and helpfully throw the error in this zone, causing us to be
         // directly called again.
-        final String treeDump = renderViewElement?.toStringDeep() ?? '<no tree>';
+        String treeDump;
+        try {
+          treeDump = renderViewElement?.toStringDeep() ?? '<no tree>';
+        } catch (exception) {
+          treeDump = '<additional error caught while dumping tree: $exception>';
+        }
         final StringBuffer expectLine = new StringBuffer();
         final int stackLinesToOmit = reportExpectCall(stack, expectLine);
         FlutterError.reportError(new FlutterErrorDetails(

packages/flutter_test/lib/src/matchers.dart

@@ -66,6 +66,17 @@ const Matcher isNotInCard = const _IsNotInCard();
 /// empty.
 const Matcher hasOneLineDescription = const _HasOneLineDescription();
 
+/// Asserts that two [double]s are equal, within some tolerated error.
+///
+/// Two values are considered equal if the difference between them is within
+/// 1e-10 of the larger one. This is an arbitrary value which can be adjusted
+/// using the `epsilon` argument. This matcher is intended to compare floating
+/// point numbers that are the result of different sequences of operations, such
+/// that they may have accumulated slightly different errors.
+Matcher moreOrLessEquals(double value, { double epsilon: 1e-10 }) {
+  return new _MoreOrLessEquals(value, epsilon);
+}
+
 class _FindsWidgetMatcher extends Matcher {
   const _FindsWidgetMatcher(this.min, this.max);
 
@@ -237,3 +248,23 @@ class _HasOneLineDescription extends Matcher {
   @override
   Description describe(Description description) => description.add('one line description');
 }
+
+class _MoreOrLessEquals extends Matcher {
+  const _MoreOrLessEquals(this.value, this.epsilon);
+
+  final double value;
+  final double epsilon;
+
+  @override
+  bool matches(Object object, Map<dynamic, dynamic> matchState) {
+    if (object is! double)
+      return false;
+    if (object == value)
+      return true;
+    final double test = object;
+    return (test - value).abs() <= epsilon;
+  }
+
+  @override
+  Description describe(Description description) => description.add('$value (±$epsilon)');
+}
\ No newline at end of file

packages/flutter_test/test/matchers_test.dart

+// Copyright 2016 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_test/flutter_test.dart';
+
+void main() {
+  test('moreOrLessEquals', () {
+    expect(0.0, moreOrLessEquals(1e-11));
+    expect(1e-11, moreOrLessEquals(0.0));
+    expect(-1e-11, moreOrLessEquals(0.0));
+
+    expect(0.0, isNot(moreOrLessEquals(1e11)));
+    expect(1e11, isNot(moreOrLessEquals(0.0)));
+    expect(-1e11, isNot(moreOrLessEquals(0.0)));
+
+    expect(0.0, isNot(moreOrLessEquals(1.0)));
+    expect(1.0, isNot(moreOrLessEquals(0.0)));
+    expect(-1.0, isNot(moreOrLessEquals(0.0)));
+
+    expect(1e-11, moreOrLessEquals(-1e-11));
+    expect(-1e-11, moreOrLessEquals(1e-11));
+
+    expect(11.0, isNot(moreOrLessEquals(-11.0, epsilon: 1.0)));
+    expect(-11.0, isNot(moreOrLessEquals(11.0, epsilon: 1.0)));
+
+    expect(11.0, moreOrLessEquals(-11.0, epsilon: 100.0));
+    expect(-11.0, moreOrLessEquals(11.0, epsilon: 100.0));
+  });
+}