Don't mangle private names.

test/kernel/regression/super_mixin.dart.txt

Index: test/kernel/regression/super_mixin.dart.txt
diff --git a/test/kernel/regression/super_mixin.dart.txt b/test/kernel/regression/super_mixin.dart.txt
index 05da0332692b77233e1f936fc8fa7a3c604aeff3..a320b58cd4c38cd036400ccc95b107cbe28f401d 100644
--- a/test/kernel/regression/super_mixin.dart.txt
+++ b/test/kernel/regression/super_mixin.dart.txt
@@ -35,11 +35,11 @@ abstract class Super+Mixin#0<T  extends  core::Object> extends self::Super<dynam
   constructor •() → self::Super<dynamic>
     : self::Super+Mixin#0::x = mix::f(), super self::Super::•();
   method publicMethod() → dynamic
-    return this.test.mixin_library::_privateMethod();
-  method test.mixin_library::_privateMethod() → dynamic
+    return this._privateMethod();
+  method _privateMethod() → dynamic
     return 49;
   method l() → dynamic
-    return mix::test.mixin_library::_private();
+    return mix::_private();
   method h() → dynamic
     return mix::V();
   method g(self::Super+Mixin#0::T a) → self::Super+Mixin#0::T
@@ -55,11 +55,11 @@ abstract class Super+Mixin#1<T  extends  core::Object, V  extends  core::Object>
   constructor •() → self::Super<self::Super+Mixin#1::V>
     : self::Super+Mixin#1::x = mix::f(), super self::Super::•();
   method publicMethod() → dynamic
-    return this.test.mixin_library::_privateMethod();
-  method test.mixin_library::_privateMethod() → dynamic
+    return this._privateMethod();
+  method _privateMethod() → dynamic
     return 49;
   method l() → dynamic
-    return mix::test.mixin_library::_private();
+    return mix::_private();
   method h() → dynamic
     return mix::V();
   method g(self::Super+Mixin#1::T a) → self::Super+Mixin#1::T
@@ -75,11 +75,11 @@ abstract class Super+Mixin#2<T  extends  core::Object> extends self::Super<dynam
   constructor •() → self::Super<dynamic>
     : self::Super+Mixin#2::x = mix::f(), super self::Super::•();
   method publicMethod() → dynamic
-    return this.test.mixin_library::_privateMethod();
-  method test.mixin_library::_privateMethod() → dynamic
+    return this._privateMethod();
+  method _privateMethod() → dynamic
     return 49;
   method l() → dynamic
-    return mix::test.mixin_library::_private();
+    return mix::_private();
   method h() → dynamic
     return mix::V();
   method g(self::Super+Mixin#2::T a) → self::Super+Mixin#2::T
@@ -95,11 +95,11 @@ abstract class Super+Mixin#3<T  extends  core::Object, V  extends  core::Object>
   constructor •() → self::Super<self::Super+Mixin#3::V>
     : self::Super+Mixin#3::x = mix::f(), super self::Super::•();
   method publicMethod() → dynamic
-    return this.test.mixin_library::_privateMethod();
-  method test.mixin_library::_privateMethod() → dynamic
+    return this._privateMethod();
+  method _privateMethod() → dynamic
     return 49;
   method l() → dynamic
-    return mix::test.mixin_library::_private();
+    return mix::_private();
   method h() → dynamic
     return mix::V();
   method g(self::Super+Mixin#3::T a) → self::Super+Mixin#3::T