The intrinsic size checks were very slow

Now we run them once per render object.

Fixes #2487

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

@@ -653,6 +653,9 @@ abstract class RenderBox extends RenderObject {
   @override
   BoxConstraints get constraints => super.constraints;
 
+  // We check the intrinsic sizes of each render box once by default.
+  bool _debugNeedsIntrinsicSizeCheck = true;
+
   @override
   void debugAssertDoesMeetConstraints() {
     assert(constraints != null);
@@ -716,6 +719,7 @@ abstract class RenderBox extends RenderObject {
           'your fault. Contact support: https://github.com/flutter/flutter/issues/new'
         );
       }
+      if (_debugNeedsIntrinsicSizeCheck || debugCheckIntrinsicSizes) {
         // verify that the intrinsics are also within the constraints
         assert(!RenderObject.debugCheckingIntrinsics);
         RenderObject.debugCheckingIntrinsics = true;
@@ -743,6 +747,7 @@ abstract class RenderBox extends RenderObject {
           failureCount += 1;
         }
         RenderObject.debugCheckingIntrinsics = false;
+        _debugNeedsIntrinsicSizeCheck = false;
         if (failures.isNotEmpty) {
           assert(failureCount > 0);
           throw new FlutterError(
@@ -754,6 +759,7 @@ abstract class RenderBox extends RenderObject {
             'your fault. Contact support: https://github.com/flutter/flutter/issues/new'
           );
         }
+      }
       return true;
     });
   }

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

@@ -68,6 +68,9 @@ bool debugPrintMarkNeedsPaintStacks = false;
 /// Log the call stacks that mark render objects as needing layout.
 bool debugPrintMarkNeedsLayoutStacks = false;
 
+/// Check the intrinsic sizes of each [RenderBox] during layout.
+bool debugCheckIntrinsicSizes = false;
+
 List<String> debugDescribeTransform(Matrix4 transform) {
   List<String> matrix = transform.toString().split('\n').map((String s) => '  $s').toList();
   matrix.removeLast();