Remove ParentData#merge

There aren't any clients anymore.

Fixes #1684

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

@@ -11,12 +11,6 @@ class MultiChildLayoutParentData extends ContainerBoxParentDataMixin<RenderBox>
   /// An object representing the identity of this child.
   Object id;
 
-  void merge(MultiChildLayoutParentData other) {
-    if (other.id != null)
-      id = other.id;
-    super.merge(other);
-  }
-
   String toString() => '${super.toString()}; id=$id';
 }
 

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

@@ -17,12 +17,6 @@ class FlexParentData extends ContainerBoxParentDataMixin<RenderBox> {
   /// according to the flex factors of the flexible children.
   int flex;
 
-  void merge(FlexParentData other) {
-    if (other.flex != null)
-      flex = other.flex;
-    super.merge(other);
-  }
-
   String toString() => '${super.toString()}; flex=$flex';
 }
 

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

@@ -291,12 +291,6 @@ class GridParentData extends ContainerBoxParentDataMixin<RenderBox> {
   /// Opaque data passed to the getChildPlacement method of the grid's [GridDelegate].
   Object placementData;
 
-  void merge(GridParentData other) {
-    if (other.placementData != null)
-      placementData = other.placementData;
-    super.merge(other);
-  }
-
   String toString() => '${super.toString()}; placementData=$placementData';
 }
 

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

@@ -30,12 +30,6 @@ class ParentData {
   /// Called when the RenderObject is removed from the tree.
   void detach() { }
 
-  /// Override this function in subclasses to merge in data from other instance
-  /// into this instance.
-  void merge(ParentData other) {
-    assert(other.runtimeType == this.runtimeType);
-  }
-
   String toString() => '<none>';
 }
 

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

@@ -164,22 +164,6 @@ class StackParentData extends ContainerBoxParentDataMixin<RenderBox> {
     left = value.left;
   }
 
-  void merge(StackParentData other) {
-    if (other.top != null)
-      top = other.top;
-    if (other.right != null)
-      right = other.right;
-    if (other.bottom != null)
-      bottom = other.bottom;
-    if (other.left != null)
-      left = other.left;
-    if (other.width != null)
-      width = other.width;
-    if (other.height != null)
-      height = other.height;
-    super.merge(other);
-  }
-
   /// Whether this child is considered positioned
   ///
   /// A child is positioned if any of the top, right, bottom, or left offsets