Don't recreate CoreTypes in transformers. Pass it in.

pkg/front_end/lib/src/fasta/kernel/kernel_target.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
index 52e264f660f745a646e839a09cd4f0a311c19075..88cea4b0b140dcd2ade6ba3506206afe400f63ec 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
@@ -40,7 +40,6 @@ import 'package:kernel/ast.dart'
         VariableGet,
         VoidType;
 
-import 'package:kernel/core_types.dart' show CoreTypes;
 import 'package:kernel/transformations/erasure.dart' show Erasure;
 
 import 'package:kernel/transformations/continuation.dart' as transformAsync;
@@ -660,7 +659,7 @@ class KernelTarget extends TargetImplementation {
   }
 
   void transformMixinApplications() {
-    mix.transformLibraries(backendTarget, loader.libraries);
+    mix.transformLibraries(backendTarget, loader.coreTypes, loader.libraries);
     ticker.logMs("Transformed mixin applications");
   }
 
@@ -670,9 +669,8 @@ class KernelTarget extends TargetImplementation {
       _program.accept(new Erasure());
       ticker.logMs("Erased type variables in generic methods");
     }
-    var coreTypes = new CoreTypes(_program);
     // TODO(kmillikin): Make this run on a per-method basis.
-    transformAsync.transformLibraries(coreTypes, loader.libraries);
+    transformAsync.transformLibraries(loader.coreTypes, loader.libraries);
     ticker.logMs("Transformed async methods");
   }
 
@@ -699,7 +697,7 @@ class KernelTarget extends TargetImplementation {
     // TODO(sigmund): replace this step with data that is directly computed from
     // the builders: we should know the tree-shaking roots without having to do
     // a second visit over the tree.
-    new RootsMarker(data).run(_program, isIncluded);
+    new RootsMarker(loader.coreTypes, data).run(_program, isIncluded);
     trimProgram(_program, data, isIncluded);
   }