Put HashMap class in dart namespace.

runtime/platform/hashmap.cc

Index: runtime/platform/hashmap.cc
diff --git a/runtime/platform/hashmap.cc b/runtime/platform/hashmap.cc
index 3bd56f836049b744cc3e9eb5e8192c6fa50b631f..50c0c244bf8940ae87b093dd43b02d0bcdd8f94b 100644
--- a/runtime/platform/hashmap.cc
+++ b/runtime/platform/hashmap.cc
@@ -7,8 +7,9 @@
 #include "bin/builtin.h"
 #include "platform/utils.h"
 
-HashMap::HashMap(MatchFun match,
-                 uint32_t initial_capacity) {
+namespace dart {
+
+HashMap::HashMap(MatchFun match, uint32_t initial_capacity) {
   match_ = match;
   Initialize(initial_capacity);
 }
@@ -188,3 +189,5 @@ void HashMap::Resize() {
   // Delete old map.
   free(map);
 }
+
+}  // namespace dart