Fix incorrect identifiers.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//2015623003 .
diff --git a/lib/src/functions.dart b/lib/src/functions.dart
index 7f3ff99..9c973b8 100644
--- a/lib/src/functions.dart
+++ b/lib/src/functions.dart
@@ -75,7 +75,7 @@
       minOrderBy = elementOrderBy;
     }
   }
-  return min;
+  return minValue;
 }
 
 /// Returns the element of [values] for which [orderBy] returns the maximum
@@ -97,7 +97,7 @@
       maxOrderBy = elementOrderBy;
     }
   }
-  return max;
+  return maxValue;
 }
 
 /// Returns the [transitive closure][] of [graph].