Work around for private names in mixin applications.

test/kernel/regression/issue_000081.dart.txt

 library;
 import self as self;
 import "dart:core" as core;
 
 class Base extends core::Object {
   field core::int hashCode;
   constructor •() → self::Base
     : self::Base::hashCode = 42, super core::Object::•();
 }
 class Sub extends self::Base {
-  field core::int _hashCode;
+  field core::int issue_000081.dart::_hashCode;
   constructor •() → self::Sub
-    : self::Sub::_hashCode = null, super self::Base::•();
+    : self::Sub::issue_000081.dart::_hashCode = null, super self::Base::•();
   get hashCode() → dynamic
-    return let final dynamic #t1 = this._hashCode in #t1.==(null) ? this._hashCode = super.self::Base::hashCode : #t1;
+    return let final dynamic #t1 = this.issue_000081.dart::_hashCode in #t1.==(null) ? this.issue_000081.dart::_hashCode = super.self::Base::hashCode : #t1;
   method foo() → dynamic {
-    this._hashCode.==(null) ? this._hashCode = super.self::Base::hashCode : null;
+    this.issue_000081.dart::_hashCode.==(null) ? this.issue_000081.dart::_hashCode = super.self::Base::hashCode : null;
   }
 }
 static method main() → dynamic {
   core::print(new self::Sub::•().hashCode);
   dynamic l = <dynamic>[null];
   let final dynamic #t2 = l in let final dynamic #t3 = 0 in #t2.[](#t3).==(null) ? #t2.[]=(#t3, "fisk") : null;
 }