Extract ClassHierarchyImpl implementation from ClassHierarchy.

pkg/kernel/lib/transformations/treeshaker.dart

Index: pkg/kernel/lib/transformations/treeshaker.dart
diff --git a/pkg/kernel/lib/transformations/treeshaker.dart b/pkg/kernel/lib/transformations/treeshaker.dart
index 1e762bda7155dedcc4143b229e91320ee1303820..9a226dbd0c995699c798058ad92316bc8786a352 100644
--- a/pkg/kernel/lib/transformations/treeshaker.dart
+++ b/pkg/kernel/lib/transformations/treeshaker.dart
@@ -89,7 +89,7 @@ class ProgramRoot {
 // TODO(asgerf): Tree shake unused instance fields.
 class TreeShaker {
   final Program program;
-  final ClassHierarchy hierarchy;
+  final ClassHierarchyImpl hierarchy;
   final CoreTypes coreTypes;
   final bool strongMode;
   final List<ProgramRoot> programRoots;
@@ -208,8 +208,8 @@ class TreeShaker {
     new _TreeShakingTransformer(this).transform(program);
   }
 
-  TreeShaker._internal(this.program, ClassHierarchy hierarchy, this.coreTypes,
-      this.strongMode, this.programRoots)
+  TreeShaker._internal(this.program, ClassHierarchyImpl hierarchy,
+      this.coreTypes, this.strongMode, this.programRoots)
       : this.hierarchy = hierarchy,
         this._dispatchedNames = new List<Set<Name>>(hierarchy.classes.length),
         this._usedMembersWithHost =