Ensure the substitution map in the CloneVisitor is mutable.

lib/clone.dart

 // Copyright (c) 2016, 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 kernel.clone;
 
 import 'ast.dart';
 import 'type_algebra.dart';
 
 /// Visitor that return a clone of a tree, maintaining references to cloned
 /// objects.
 ///
 /// It is safe to clone members, but cloning a class or library is not
 /// supported.
 class CloneVisitor extends TreeVisitor {
   final Map<VariableDeclaration, VariableDeclaration> variables =
       <VariableDeclaration, VariableDeclaration>{};
   final Map<LabeledStatement, LabeledStatement> labels =
       <LabeledStatement, LabeledStatement>{};
   final Map<SwitchCase, SwitchCase> switchCases = <SwitchCase, SwitchCase>{};
   final Map<TypeParameter, DartType> typeSubstitution;
 
   CloneVisitor({Map<TypeParameter, DartType> typeSubstitution})
-      : this.typeSubstitution = typeSubstitution ?? <TypeParameter, DartType>{};
+      : this.typeSubstitution = ensureMutable(typeSubstitution);
+
+  static Map<TypeParameter, DartType> ensureMutable(
+      Map<TypeParameter, DartType> map) {
+    // We need to mutate this map, so make sure we don't use a constant map.
+    if (map == null || map.isEmpty) {
+      return <TypeParameter, DartType>{};
+    }
+    return map;
+  }
 
   TreeNode visitLibrary(Library node) {
     throw 'Cloning of libraries is not implemented';
   }
 
   TreeNode visitClass(Class node) {
     throw 'Cloning of classes is not implemented';
   }
 
   TreeNode clone(TreeNode node) =>
@@ skipping 347 matching lines @@
   visitArguments(Arguments node) {
     return new Arguments(node.positional.map(clone).toList(),
         types: node.types.map(visitType).toList(),
         named: node.named.map(clone).toList());
   }
 
   visitNamedExpression(NamedExpression node) {
     return new NamedExpression(node.name, clone(node.value));
   }
 }