Run tree shaking in strong-mode baseline tests.

pkg/kernel/testcases/strong-mode/micro.baseline.txt

 library;
 import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
   method instanceMethod() → dynamic {
     return 123;
   }
 }
 abstract class ExternalValue extends core::Object {
-  constructor •() → void
-    : super core::Object::•()
-    ;
 }
 abstract class Bar extends core::Object {
-  constructor •() → void
-    : super core::Object::•()
-    ;
   abstract method externalInstanceMethod() → self::ExternalValue;
 }
 class Box extends core::Object {
   field dynamic field = null;
   constructor •() → void
     : super core::Object::•()
     ;
 }
 class FinalBox extends core::Object {
   final field dynamic finalField;
@@ skipping 40 matching lines @@
   dynamic a = box.{self::Box::field};
   self::FinalBox finalBox = new self::FinalBox::•("dfg");
   dynamic b = finalBox.{self::FinalBox::finalField};
   self::SubFinalBox subBox = new self::SubFinalBox::•("dfg");
   dynamic c = subBox.{self::FinalBox::finalField};
   self::makeDynamicCall(new self::DynamicReceiver1::•());
   self::makeDynamicCall(new self::DynamicReceiver2::•());
   core::List<core::String> list = <core::String>["string"];
   core::String d = list.{core::List::[]}(0);
 }