Remove the second argument to MultiChildLayoutDelegate.performLayout()

Fixes https://github.com/flutter/flutter/issues/2403

I have an e-mail ready to send to flutter-dev about this.

packages/flutter/lib/src/material/scaffold.dart

@@ -39,9 +39,8 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
 
   final EdgeDims padding;
 
-  void performLayout(Size size, BoxConstraints constraints) {
-
-    BoxConstraints looseConstraints = constraints.loosen();
+  void performLayout(Size size) {
+    BoxConstraints looseConstraints = new BoxConstraints.loose(size);
 
     // This part of the layout has the same effect as putting the toolbar and
     // body in a column and making the body flexible. What's different is that

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

@@ -103,7 +103,7 @@ abstract class MultiChildLayoutDelegate {
     return '${childParentData.id}: $child';
   }
 
-  void _callPerformLayout(Size size, BoxConstraints constraints, RenderBox firstChild) {
+  void _callPerformLayout(Size size, RenderBox firstChild) {
     // A particular layout delegate could be called reentrantly, e.g. if it used
     // by both a parent and a child. So, we must restore the _idToChild map when
     // we return.
@@ -138,7 +138,7 @@ abstract class MultiChildLayoutDelegate {
         });
         child = childParentData.nextSibling;
       }
-      performLayout(size, constraints);
+      performLayout(size);
       assert(() {
         if (_debugChildrenNeedingLayout.isNotEmpty) {
           if (_debugChildrenNeedingLayout.length > 1) {
@@ -176,11 +176,10 @@ abstract class MultiChildLayoutDelegate {
   /// possible given the constraints.
   Size getSize(BoxConstraints constraints) => constraints.biggest;
 
-  /// Override this method to lay out and position all children given
-  /// this widget's size and the specified constraints. This method
-  /// must call [layoutChild] for each child. It should also specify
-  /// the final position of each child with [positionChild].
-  void performLayout(Size size, BoxConstraints constraints);
+  /// Override this method to lay out and position all children given this
+  /// widget's size. This method must call [layoutChild] for each child. It
+  /// should also specify the final position of each child with [positionChild].
+  void performLayout(Size size);
 
   /// Override this method to return true when the children need to be
   /// laid out. This should compare the fields of the current delegate
@@ -257,7 +256,7 @@ class RenderCustomMultiChildLayoutBox extends RenderBox
   }
 
   void performLayout() {
-    delegate._callPerformLayout(size, constraints, firstChild);
+    delegate._callPerformLayout(size, firstChild);
   }
 
   void paint(PaintingContext context, Offset offset) {

packages/flutter/test/widget/custom_multi_child_layout_test.dart

@@ -17,16 +17,15 @@ class TestMultiChildLayoutDelegate extends MultiChildLayoutDelegate {
   }
 
   Size performLayoutSize;
-  BoxConstraints performLayoutConstraints;
   Size performLayoutSize0;
   Size performLayoutSize1;
   bool performLayoutIsChild;
 
-  void performLayout(Size size, BoxConstraints constraints) {
+  void performLayout(Size size) {
     assert(!RenderObject.debugCheckingIntrinsics);
     expect(() {
       performLayoutSize = size;
-      performLayoutConstraints = constraints;
+      BoxConstraints constraints = new BoxConstraints.loose(size);
       performLayoutSize0 = layoutChild(0, constraints);
       performLayoutSize1 = layoutChild(1, constraints);
       performLayoutIsChild = isChild('fred');
@@ -68,10 +67,6 @@ void main() {
 
       expect(delegate.performLayoutSize.width, 200.0);
       expect(delegate.performLayoutSize.height, 300.0);
-      expect(delegate.performLayoutConstraints.minWidth, 0.0);
-      expect(delegate.performLayoutConstraints.maxWidth, 800.0);
-      expect(delegate.performLayoutConstraints.minHeight, 0.0);
-      expect(delegate.performLayoutConstraints.maxHeight, 600.0);
       expect(delegate.performLayoutSize0.width, 150.0);
       expect(delegate.performLayoutSize0.height, 100.0);
       expect(delegate.performLayoutSize1.width, 100.0);