A type should be a subtype of Function if its class declares a call function,

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 1168 matching lines @@
   // TODO(koda): Unite w/ hash table.
   RawArray* functions() const { return raw_ptr()->functions_; }
   void SetFunctions(const Array& value) const;
   void AddFunction(const Function& function) const;
   void RemoveFunction(const Function& function) const;
   RawFunction* FunctionFromIndex(intptr_t idx) const;
   intptr_t FindImplicitClosureFunctionIndex(const Function& needle) const;
   RawFunction* ImplicitClosureFunctionFromIndex(intptr_t idx) const;
 
   RawFunction* LookupDynamicFunction(const String& name) const;
+  RawFunction* LookupDynamicFunctionAllowAbstract(const String& name) const;
   RawFunction* LookupDynamicFunctionAllowPrivate(const String& name) const;
   RawFunction* LookupStaticFunction(const String& name) const;
   RawFunction* LookupStaticFunctionAllowPrivate(const String& name) const;
   RawFunction* LookupConstructor(const String& name) const;
   RawFunction* LookupConstructorAllowPrivate(const String& name) const;
   RawFunction* LookupFactory(const String& name) const;
   RawFunction* LookupFactoryAllowPrivate(const String& name) const;
   RawFunction* LookupFunction(const String& name) const;
   RawFunction* LookupFunctionAllowPrivate(const String& name) const;
   RawFunction* LookupGetterFunction(const String& name) const;
@@ skipping 167 matching lines @@
   void set_cha_codes(const Array& value) const;
 
   bool TraceAllocation(Isolate* isolate) const;
   void SetTraceAllocation(bool trace_allocation) const;
 
  private:
   enum MemberKind {
     kAny = 0,
     kStatic,
     kInstance,
+    kInstanceAllowAbstract,
     kConstructor,
     kFactory,
   };
   enum StateBits {
     kConstBit = 0,
     kImplementedBit = 1,
     kTypeFinalizedBit = 2,
     kClassFinalizedPos = 3,
     kClassFinalizedSize = 2,
     kAbstractBit = kClassFinalizedPos + kClassFinalizedSize,  // = 5
@@ skipping 922 matching lines @@
 
   static const char* KindToCString(RawFunction::Kind kind);
 
   bool IsGenerativeConstructor() const {
     return (kind() == RawFunction::kConstructor) && !is_static();
   }
   bool IsImplicitConstructor() const;
   bool IsFactory() const {
     return (kind() == RawFunction::kConstructor) && is_static();
   }
-  bool IsDynamicFunction() const {
-    if (is_static() || is_abstract()) {
+  bool IsDynamicFunction(bool allow_abstract = false) const {
+    if (is_static() || (!allow_abstract && is_abstract())) {
       return false;
     }
     switch (kind()) {
       case RawFunction::kRegularFunction:
       case RawFunction::kGetterFunction:
       case RawFunction::kSetterFunction:
       case RawFunction::kImplicitGetter:
       case RawFunction::kImplicitSetter:
       case RawFunction::kMethodExtractor:
       case RawFunction::kNoSuchMethodDispatcher:
@@ skipping 6115 matching lines @@
 
 
 RawObject* MegamorphicCache::GetTargetFunction(const Array& array,
                                                intptr_t index) {
   return array.At((index * kEntryLength) + kTargetFunctionIndex);
 }
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_H_