Store named parameters in sorted lists instead of using maps.

lib/type_environment.dart

Index: lib/type_environment.dart
diff --git a/lib/type_environment.dart b/lib/type_environment.dart
index 96f6a21a5f9847f98b411e8716f46199fb4ba589..c354c44b7048b5bac5aa22f4942d0b7e57ac3588 100644
--- a/lib/type_environment.dart
+++ b/lib/type_environment.dart
@@ -206,12 +206,17 @@ abstract class SubtypeTester {
         return false;
       }
     }
-    for (String name in supertype.namedParameters.keys) {
-      var supertypeParameter = supertype.namedParameters[name];
-      var subtypeParameter = subtype.namedParameters[name];
-      if (subtypeParameter == null) return false;
+    int subtypeNameIndex = 0;
+    for (NamedType supertypeParameter in supertype.namedParameters) {
+      while (subtypeNameIndex < subtype.namedParameters.length &&
+          subtype.namedParameters[subtypeNameIndex].name !=
+              supertypeParameter.name) {
+        ++subtypeNameIndex;
+      }
+      if (subtypeNameIndex == subtype.namedParameters.length) return false;
+      NamedType subtypeParameter = subtype.namedParameters[subtypeNameIndex];
       // Termination: Both types shrink in size.
-      if (!isSubtypeOf(supertypeParameter, subtypeParameter)) {
+      if (!isSubtypeOf(supertypeParameter.type, subtypeParameter.type)) {
         return false;
       }
     }