Put HashMap class in dart namespace.

runtime/bin/hashmap_test.cc

Index: runtime/bin/hashmap_test.cc
diff --git a/runtime/bin/hashmap_test.cc b/runtime/bin/hashmap_test.cc
index 23d4ad83fb120dc6d6c4bcc3978eb3f29e252476..b51d1cfb0ff7701616a14f3142e391f68bfe4fd0 100644
--- a/runtime/bin/hashmap_test.cc
+++ b/runtime/bin/hashmap_test.cc
@@ -7,6 +7,8 @@
 #include "platform/hashmap.h"
 #include "vm/unit_test.h"
 
+namespace dart {
+
 // Default initial size of hashmaps used in these tests.
 static intptr_t kInitialSize = 8;
 
@@ -167,3 +169,5 @@ UNIT_TEST_CASE(Set) {
   TestSet(CollisionHash3, 50);
   TestSet(CollisionHash4, 50);
 }
+
+}  // namespace dart