Move maxBy into the MaterialArc file (#5905)

As per our style guide, if you can't explain something, refactor it. I
studied this function for a while and still can't explain it, so...

packages/flutter/lib/src/foundation/basic_types.dart

@@ -243,21 +243,3 @@ class _LazyListIterator<E> implements Iterator<E> {
     return true;
   }
 }
-
-// COLLECTION UTILITIES
-
-typedef dynamic KeyFunc<T>(T input);
-
-/// Select the element for which the key function returns the maximum value.
-dynamic/*=T*/ maxBy/*<T>*/(Iterable<dynamic/*=T*/> input, KeyFunc/*<T>*/ keyFunc) {
-  dynamic/*=T*/ maxValue;
-  dynamic maxKey;
-  for (dynamic/*=T*/ value in input) {
-    dynamic key = keyFunc(value);
-    if (maxKey == null || key > maxKey) {
-      maxValue = value;
-      maxKey = key;
-    }
-  }
-  return maxValue;
-}

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

@@ -5,7 +5,6 @@
 import 'dart:math' as math;
 import 'dart:ui' show hashValues, lerpDouble;
 
-import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:meta/meta.dart';
 
@@ -160,6 +159,22 @@ const List<_Diagonal> _allDiagonals = const <_Diagonal>[
   const _Diagonal(_CornerId.bottomLeft, _CornerId.topRight),
 ];
 
+typedef dynamic _KeyFunc<T>(T input);
+
+// Select the element for which the key function returns the maximum value.
+dynamic/*=T*/ _maxBy/*<T>*/(Iterable<dynamic/*=T*/> input, _KeyFunc/*<T>*/ keyFunc) {
+  dynamic/*=T*/ maxValue;
+  dynamic maxKey;
+  for (dynamic/*=T*/ value in input) {
+    dynamic key = keyFunc(value);
+    if (maxKey == null || key > maxKey) {
+      maxValue = value;
+      maxKey = key;
+    }
+  }
+  return maxValue;
+}
+
 /// A [Tween] that animates a [Rect] from [begin] to [end].
 ///
 /// The rectangle corners whose diagonal is closest to the overall direction of
@@ -184,7 +199,7 @@ class MaterialRectArcTween extends RectTween {
     assert(begin != null);
     assert(end != null);
     final Offset centersVector = end.center - begin.center;
-    _diagonal = maxBy(_allDiagonals, (_Diagonal d) => _diagonalSupport(centersVector, d));
+    _diagonal = _maxBy/*<_Diagonal>*/(_allDiagonals, (_Diagonal d) => _diagonalSupport(centersVector, d));
     _beginArc = new MaterialPointArcTween(
       begin: _cornerFor(begin, _diagonal.beginId),
       end: _cornerFor(end, _diagonal.beginId)