Implement override-based type inference for instance methods.

pkg/front_end/testcases/inference/infer_types_on_generic_instantiations_5.dart.strong.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 abstract class I<E extends core::Object> extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
   abstract method m(dynamic a, (dynamic, self::I::E) → core::String f) → core::String;
 }
 abstract class A<E extends core::Object> extends core::Object implements self::I<self::A::E> {
   const constructor •() → void
     : super core::Object::•()
     ;
   abstract method m(dynamic a, (dynamic, self::A::E) → core::String f) → core::String;
 }
 abstract class M extends core::Object {
   final field core::int y = 0;
   constructor •() → void
     : super core::Object::•()
     ;
 }
 class B<E extends core::Object> extends self::A<self::B::E> implements self::M {
   const constructor •() → void
     : super self::A::•()
     ;
   get y() → core::int
     return 0;
-  method m(dynamic a, (dynamic, self::B::E) → dynamic f) → dynamic {}
+  method m(dynamic a, (dynamic, self::B::E) → dynamic f) → core::String {}
 }
 static method foo() → dynamic {
   core::int y = new self::B::•<dynamic>().{self::B::m}(null, null);
   core::String z = new self::B::•<dynamic>().{self::B::m}(null, null);
 }
 static method main() → dynamic {}