Merge the work on Generic Types Reification from 'dart-lang/reify' repo

pkg/kernel/lib/target/vm.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library kernel.target.vm;
 
 import '../ast.dart';
 import '../class_hierarchy.dart';
 import '../core_types.dart';
 import '../transformations/continuation.dart' as cont;
 import '../transformations/erasure.dart';
@@ skipping 56 matching lines @@
     var coreTypes = new CoreTypes(program);
 
     if (strongMode) {
       new InsertTypeChecks(hierarchy: _hierarchy, coreTypes: coreTypes)
           .transformProgram(program);
       new InsertCovarianceChecks(hierarchy: _hierarchy, coreTypes: coreTypes)
           .transformProgram(program);
     }
 
     if (flags.treeShake) {
-      new TreeShaker(program,
-              hierarchy: _hierarchy,
-              coreTypes: coreTypes,
-              strongMode: strongMode,
-              programRoots: flags.programRoots)
-          .transform(program);
-      _hierarchy = null; // Hierarchy must be recomputed.
+      performTreeShaking(program);
     }
 
     cont.transformProgram(program);
 
     // Repair `_getMainClosure()` function in dart:_builtin.
     setup_builtin_library.transformProgram(program);
 
     if (strongMode) {
       new Erasure().transform(program);
     }
 
     new SanitizeForVM().transform(program);
   }
+
+  void performTreeShaking(Program program) {
+    var coreTypes = new CoreTypes(program);
+    new TreeShaker(program,
+        hierarchy: _hierarchy,
+        coreTypes: coreTypes,
+        strongMode: strongMode,
+        programRoots: flags.programRoots)
+        .transform(program);
+    _hierarchy = null; // Hierarchy must be recomputed.
+  }
 }