Simplify the bailout test for generic inference

pkg/analyzer/lib/src/generated/type_system.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.generated.type_system;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'package:analyzer/dart/ast/token.dart' show TokenType;
@@ skipping 1408 matching lines @@
       // we choose the upper bound as being the most useful.
       //
       // Otherwise we choose the more precise lower bound.
       _TypeParameterVariance variance =
           new _TypeParameterVariance.from(typeParam, fnType.returnType);
 
       _TypeParameterBound bound = _bounds[typeParam];
       inferredTypes[i] =
           variance.passedIn || bound.lower.isBottom ? bound.upper : bound.lower;
 
-      // See if the constraints on the type variable are satisfied.
-      //
-      // If not, bail out of the analysis, unless a partial solution was
-      // requested. If we are willing to accept a partial solution, fall back to
-      // the known upper bound (if any) or `dynamic` for this unsolvable type
-      // variable.
-      if (inferredTypes[i].isBottom ||
-          !isSubtypeOf(inferredTypes[i],
-              bound.upper.substitute2(inferredTypes, fnTypeParams)) ||
-          !isSubtypeOf(bound.lower.substitute2(inferredTypes, fnTypeParams),
-              inferredTypes[i])) {
+      // See if the bounds can be satisfied.
+      if (bound.upper.isBottom ||
+          !_typeSystem.isSubtypeOf(bound.lower, bound.upper)) {
         // Inference failed. Bail.
         return null;
       }
     }
 
     // Return the instantiated type.
     return fnType.instantiate(inferredTypes);
   }
 
   @override
@@ skipping 239 matching lines @@
 }
 
 bool _isBottom(DartType t, {bool dynamicIsBottom: false}) {
   return (t.isDynamic && dynamicIsBottom) || t.isBottom;
 }
 
 bool _isTop(DartType t, {bool dynamicIsBottom: false}) {
   // TODO(leafp): Document the rules in play here
   return (t.isDynamic && !dynamicIsBottom) || t.isObject;
 }