Add a new implementation of HashMap that uses JS objects for its (multiple) hash tables.

runtime/vm/bootstrap.h

Index: runtime/vm/bootstrap.h
diff --git a/runtime/vm/bootstrap.h b/runtime/vm/bootstrap.h
index a638579eefd0d343909aa3ffc400f086d1360967..fdb616df32b8cf2fb48397795752453330719b5a 100644
--- a/runtime/vm/bootstrap.h
+++ b/runtime/vm/bootstrap.h
@@ -40,6 +40,7 @@ class Bootstrap : public AllStatic {
   static const char corelib_source_[];
   static const char corelib_patch_[];
   static const char collection_source_[];
+  static const char collection_patch_[];
   static const char collection_dev_source_[];
   static const char crypto_source_[];
   static const char isolate_source_[];