Make the JSON decoder (parser) create Dart maps lazily

runtime/lib/collection_patch.dart

 // Copyright (c) 2013, 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.
 
 patch class HashMap<K, V> {
   /* patch */ factory HashMap({ bool equals(K key1, K key2),
                                 int hashCode(K key),
                                 bool isValidKey(potentialKey) }) {
     if (isValidKey == null) {
       if (hashCode == null) {
@@ skipping 156 matching lines @@
             (_modificationCount + 1) & _MODIFICATION_COUNT_MASK;
         return entry.value;
       }
       previous = entry;
       entry = next;
     }
     return null;
   }
 
   void clear() {
-    _elementCount = 0;
     _buckets = new List(_INITIAL_CAPACITY);
-    _modificationCount = (_modificationCount + 1) & _MODIFICATION_COUNT_MASK;
+    if (_elementCount > 0) {
+      _elementCount = 0;
+      _modificationCount = (_modificationCount + 1) & _MODIFICATION_COUNT_MASK;
+    }
   }
 
   void _removeEntry(_HashMapEntry entry,
                     _HashMapEntry previousInBucket,
                     int bucketIndex) {
     if (previousInBucket == null) {
       _buckets[bucketIndex] = entry.next;
     } else {
       previousInBucket.next = entry.next;
     }
@@ skipping 476 matching lines @@
 
   void removeWhere(bool test(E element)) {
     _filterWhere(test, true);
   }
 
   void retainWhere(bool test(E element)) {
     _filterWhere(test, false);
   }
 
   void clear() {
-    _elementCount = 0;
     _buckets = new List(_INITIAL_CAPACITY);
-    _modificationCount++;
+    if (_elementCount > 0) {
+      _elementCount = 0;
+      _modificationCount = (_modificationCount + 1) & _MODIFICATION_COUNT_MASK;
+    }
   }
 
   void _addEntry(E key, int hashCode, int index) {
     _buckets[index] = new _HashSetEntry(key, hashCode, _buckets[index]);
     int newElements = _elementCount + 1;
     _elementCount = newElements;
     int length = _buckets.length;
     // If we end up with more than 75% non-empty entries, we
     // resize the backing store.
     if ((newElements << 2) > ((length << 1) + length)) _resize();
@@ skipping 267 matching lines @@
       action(entry.key, entry.value);
       if (modificationCount != _modificationCount) {
         throw new ConcurrentModificationError(this);
       }
       cursor = cursor._nextEntry;
     }
   }
 
   void clear() {
     _nextEntry = _previousEntry = this;
-    _elementCount = 0;
     _buckets = new List(_HashMap._INITIAL_CAPACITY);
-    _modificationCount = (_modificationCount + 1) & _MODIFICATION_COUNT_MASK;
+    if (_elementCount > 0) {
+      _elementCount = 0;
+      _modificationCount = (_modificationCount + 1) & _MODIFICATION_COUNT_MASK;
+    }
   }
 
   void _addEntry(List buckets, int index, int length,
                  K key, V value, int hashCode) {
     _HashMapEntry entry =
         new _LinkedHashMapEntry(key, value, hashCode, buckets[index],
                                 _previousEntry, this);
     buckets[index] = entry;
     int newElements = _elementCount + 1;
     _elementCount = newElements;
@@ skipping 261 matching lines @@
       return false;
     }
     _LinkedHashSetEntry entry = _next;
     _current = entry.key;
     _next = entry._nextEntry;
     return true;
   }
 
   E get current => _current;
 }