Add 'Clear' method to HashMap/HashSet templates.

runtime/vm/hash_table.h

 // Copyright (c) 2014, 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.
 
 #ifndef VM_HASH_TABLE_H_
 #define VM_HASH_TABLE_H_
 
 // Temporarily used when sorting the indices in EnumIndexHashTable.
 // TODO(koda): Remove these dependencies before using in production.
 #include <map>
@@ skipping 544 matching lines @@
   bool Remove(const Key& key) const {
     intptr_t entry = BaseIterTable::FindKey(key);
     if (entry == -1) {
       return false;
     } else {
       BaseIterTable::DeleteEntry(entry);
       return true;
     }
   }
 
+  void Clear() const {
+    BaseIterTable::Initialize();
+  }
+
  protected:
   void EnsureCapacity() const {
     static const double kMaxLoadFactor = 0.75;
+    // We currently never shrink.
     HashTables::EnsureLoadFactor(0.0, kMaxLoadFactor, *this);
   }
 };
 
 
 template<typename KeyTraits>
 class UnorderedHashMap : public HashMap<UnorderedHashTable<KeyTraits, 1> > {
  public:
   typedef HashMap<UnorderedHashTable<KeyTraits, 1> > BaseMap;
   explicit UnorderedHashMap(RawArray* data) : BaseMap(data) {}
@@ skipping 66 matching lines @@
   bool Remove(const Key& key) const {
     intptr_t entry = BaseIterTable::FindKey(key);
     if (entry == -1) {
       return false;
     } else {
       BaseIterTable::DeleteEntry(entry);
       return true;
     }
   }
 
+  void Clear() const {
+    BaseIterTable::Initialize();
+  }
+
  protected:
   void EnsureCapacity() const {
     static const double kMaxLoadFactor = 0.75;
+    // We currently never shrink.
     HashTables::EnsureLoadFactor(0.0, kMaxLoadFactor, *this);
   }
 };
 
 
 template<typename KeyTraits>
 class UnorderedHashSet : public HashSet<UnorderedHashTable<KeyTraits, 0> > {
  public:
   typedef HashSet<UnorderedHashTable<KeyTraits, 0> > BaseSet;
   explicit UnorderedHashSet(RawArray* data) : BaseSet(data) {}
   UnorderedHashSet(Isolate* isolate, RawArray* data) : BaseSet(isolate, data) {}
 };
 
 
 template<typename KeyTraits>
 class EnumIndexHashSet : public HashSet<EnumIndexHashTable<KeyTraits, 0> > {
  public:
   typedef HashSet<EnumIndexHashTable<KeyTraits, 0> > BaseSet;
   explicit EnumIndexHashSet(RawArray* data) : BaseSet(data) {}
   EnumIndexHashSet(Isolate* isolate, RawArray* data) : BaseSet(isolate, data) {}
 };
 
 }  // namespace dart
 
 #endif  // VM_HASH_TABLE_H_