Reenable the gallery smoke test (#4303)

...and fix some grid and table intrinsic issues it uncovered.

examples/flutter_gallery/test/smoke_test.dart

@@ -109,6 +109,5 @@ void main() {
     await tester.tap(find.text('Light'));
     await tester.pump();
     await tester.pump(const Duration(seconds: 1)); // Wait until it's changed.
-  },
-  skip: true);
+  });
 }

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

@@ -292,6 +292,10 @@ abstract class GridDelegateWithInOrderChildPlacement extends GridDelegate {
 
 
 /// A [GridDelegate] that divides the grid's width evenly for a fixed number of columns.
+///
+/// Grids using this delegate cannot validly be placed inside an unconstrained
+/// horizontal space, since they attempt to divide the incoming horizontal
+/// maximum width constraint.
 class FixedColumnCountGridDelegate extends GridDelegateWithInOrderChildPlacement {
   /// Creates a grid delegate that uses a fixed column count.
   ///
@@ -349,7 +353,17 @@ class FixedColumnCountGridDelegate extends GridDelegateWithInOrderChildPlacement
     return 0.0;
   }
 
-  // TODO(ianh): Provide efficient intrinsic height functions.
+  @override
+  double getMinIntrinsicHeight(double width, int childCount) {
+    // TODO(ianh): Strictly, this should examine the children.
+    return 0.0;
+  }
+
+  @override
+  double getMaxIntrinsicHeight(double width, int childCount) {
+    // TODO(ianh): Strictly, this should examine the children.
+    return 0.0;
+  }
 }
 
 /// A [GridDelegate] that fills the width with a variable number of tiles.

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

@@ -892,7 +892,6 @@ class RenderTable extends RenderBox {
           if (flexes[x] != null) {
             final double newWidth = widths[x] - deficit * flexes[x] / totalFlex;
             assert(newWidth.isFinite);
-            assert(newWidth >= 0.0);
             if (newWidth <= minWidths[x]) {
               // shrank to minimum
               deficit -= widths[x] - minWidths[x];
@@ -904,6 +903,7 @@ class RenderTable extends RenderBox {
               widths[x] = newWidth;
               newTotalFlex += flexes[x];
             }
+            assert(widths[x] >= 0.0);
           }
         }
         totalFlex = newTotalFlex;