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 d0e7e63972c90da132e3a2ca1f56dce3550514ee..9056117f97f2e1610caef312cfaaec1755c9a670 100644
--- a/runtime/vm/bootstrap.h
+++ b/runtime/vm/bootstrap.h
@@ -41,6 +41,7 @@ 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_[];