Allow the optimizer to canonicalize instantiated function types (issue 11775).

runtime/vm/object.h

 // Copyright (c) 2012, 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.
 
 #ifndef VM_OBJECT_H_
 #define VM_OBJECT_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/utils.h"
@@ skipping 671 matching lines @@
   }
   static intptr_t signature_function_offset() {
     return OFFSET_OF(RawClass, signature_function_);
   }
 
   // Return the signature type of this signature class.
   // For example, if this class represents a signature of the form
   // 'F<T, R>(T, [b: B, c: C]) => R', then its signature type is a parameterized
   // type with this class as the type class and type parameters 'T' and 'R'
   // as its type argument vector.
+  // SignatureType is used as the type of formal parameters representing a
+  // function.
   RawType* SignatureType() const;
 
   RawLibrary* library() const { return raw_ptr()->library_; }
   void set_library(const Library& value) const;
 
   // The type parameters (and their bounds) are specified as an array of
   // TypeParameter.
   RawTypeArguments* type_parameters() const {
       return raw_ptr()->type_parameters_;
   }
@@ skipping 5303 matching lines @@
 
 
 RawObject* MegamorphicCache::GetTargetFunction(const Array& array,
                                                intptr_t index) {
   return array.At((index * kEntryLength) + kTargetFunctionIndex);
 }
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_H_