Store named parameters in sorted lists instead of using maps.

testcases/spec-mode/named_parameters.baseline.txt

+library;
+import self as self;
+import "dart:core" as core;
+
+class Superclass extends core::Object {
+  constructor •() → void
+    : super core::Object::•()
+    ;
+  method foo({dynamic alpha, dynamic beta}) → dynamic {}
+  method bar({dynamic alpha, dynamic beta}) → dynamic {}
+}
+class Subclass extends self::Superclass {
+  constructor •() → void
+    : super self::Superclass::•()
+    ;
+  method foo({dynamic alpha, dynamic beta}) → dynamic {}
+  method bar({dynamic alpha, dynamic beta}) → dynamic {}
+}
+static method topLevelNamed(dynamic beta, dynamic alpha, {dynamic delta, dynamic gamma}) → dynamic {}
+static method topLevelOptional(dynamic beta, dynamic alpha, [dynamic gamma, dynamic delta]) → dynamic {}
+static method main() → dynamic {
+  new self::Subclass::•().foo(beta: 1, alpha: 2);
+  new self::Subclass::•().foo(alpha: 1, beta: 2);
+  self::topLevelNamed(1, 2, gamma: 3, delta: 4);
+  self::topLevelNamed(1, 2, delta: 3, gamma: 4);
+}