Do not add run spacing if there are no run metrics (#16290)

* do not add run spacing if there are no run metrics

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

@@ -629,7 +629,9 @@ class RenderWrap extends RenderBox with ContainerRenderObjectMixin<RenderBox, Wr
     }
     if (childCount > 0) {
       mainAxisExtent = math.max(mainAxisExtent, runMainAxisExtent);
-      crossAxisExtent += runCrossAxisExtent + runSpacing;
+      crossAxisExtent += runCrossAxisExtent;
+      if (runMetrics.isNotEmpty)
+        crossAxisExtent += runSpacing;
       runMetrics.add(new _RunMetrics(runMainAxisExtent, runCrossAxisExtent, childCount));
     }
 

packages/flutter/test/widgets/wrap_test.dart

@@ -710,7 +710,7 @@ void main() {
         ),
       ),
     );
-    expect(tester.renderObject<RenderBox>(find.byType(Wrap)).size, equals(const Size(270.0, 258.0)));
+    expect(tester.renderObject<RenderBox>(find.byType(Wrap)).size, equals(const Size(270.0, 250.0)));
     verify(tester, <Offset>[
       const Offset(0.0, 0.0),
       const Offset(22.0, 0.0),
@@ -853,17 +853,46 @@ void main() {
   });
 
   testWidgets('Object exactly matches container width', (WidgetTester tester) async {
-    await tester.pumpWidget(new Wrap(
-      direction: Axis.horizontal,
-      textDirection: TextDirection.ltr,
-      spacing: 10.0,
-      runSpacing: 10.0,
-      children: const <Widget>[
-        const SizedBox(width: 800.0, height: 0.0),
-      ],
-    ));
+    await tester.pumpWidget(
+      new Column(
+        children: <Widget>[
+          new Wrap(
+            direction: Axis.horizontal,
+            textDirection: TextDirection.ltr,
+            spacing: 10.0,
+            runSpacing: 10.0,
+            children: const <Widget>[
+              const SizedBox(width: 800.0, height: 10.0),
+            ],
+          ),
+        ],
+      )
+    );
 
-    expect(tester.renderObject<RenderBox>(find.byType(Wrap)).size, equals(const Size(800.0, 600.0)));
+    expect(tester.renderObject<RenderBox>(find.byType(Wrap)).size, equals(const Size(800.0, 10.0)));
     verify(tester, <Offset>[const Offset(0.0, 0.0)]);
+
+    await tester.pumpWidget(
+      new Column(
+        children: <Widget>[
+          new Wrap(
+            direction: Axis.horizontal,
+            textDirection: TextDirection.ltr,
+            spacing: 10.0,
+            runSpacing: 10.0,
+            children: const <Widget>[
+              const SizedBox(width: 800.0, height: 10.0),
+              const SizedBox(width: 800.0, height: 10.0),
+            ],
+          ),
+        ],
+      )
+    );
+
+    expect(tester.renderObject<RenderBox>(find.byType(Wrap)).size, equals(const Size(800.0, 30.0)));
+    verify(tester, <Offset>[
+      const Offset(0.0, 0.0),
+      const Offset(0.0, 20.0),
+    ]);
   });
 }