Fix formatting of Flex error message (#16498)

packages/flutter/lib/src/rendering/flex.dart

@@ -681,7 +681,7 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
             if (node != null) {
               information.writeln('The nearest ancestor providing an unbounded width constraint is:');
               information.write('  ');
-              information.write(node.toStringShallow(joiner: '\n  '));
+              information.writeln(node.toStringShallow(joiner: '\n  '));
             }
             information.writeln('See also: https://flutter.io/layout/');
             addendum = information.toString();

packages/flutter/test/widgets/flex_test.dart

@@ -110,4 +110,34 @@ void main() {
       ),
     );
   });
+
+  testWidgets('Error information is printed correctly', (WidgetTester tester) async {
+    // We run this twice, the first time without an error, so that the second time
+    // we only get a single exception. Otherwise we'd get two, the one we want and
+    // an extra one when we discover we never computed a size.
+    await tester.pumpWidget(
+      new Column(
+        children: <Widget>[
+          new Column(),
+        ],
+      ),
+      Duration.zero,
+      EnginePhase.layout,
+    );
+    await tester.pumpWidget(
+      new Column(
+        children: <Widget>[
+          new Column(
+            children: <Widget>[
+              new Expanded(child: new Container()),
+            ],
+          ),
+        ],
+      ),
+      Duration.zero,
+      EnginePhase.layout,
+    );
+    final String message = tester.takeException().toString();
+    expect(message, contains('\nSee also:'));
+  });
 }