Rationalize coercions

test/dart_codegen/expect/collection/maps.dart

 part of dart.collection;
  abstract class MapBase<K, V> = Object with MapMixin<K, V>;
  abstract class MapMixin<K, V> implements Map<K, V> {Iterable<K> get keys;
  V operator [](Object key);
  operator []=(K key, V value);
  V remove(Object key);
  void clear();
  void forEach(void action(K key, V value)) {
   for (K key in keys) {
     action(key, this[key]);
@@ skipping 22 matching lines @@
  bool get isNotEmpty => keys.isNotEmpty;
  Iterable<V> get values => new _MapBaseValueIterable<V>(this);
  String toString() => Maps.mapToString(this);
 }
  abstract class UnmodifiableMapBase<K, V> = MapBase<K, V> with _UnmodifiableMapMixin<K, V>;
  class _MapBaseValueIterable<V> extends IterableBase<V> implements EfficientLength {final Map _map;
  _MapBaseValueIterable(this._map);
  int get length => _map.length;
  bool get isEmpty => _map.isEmpty;
  bool get isNotEmpty => _map.isNotEmpty;
- V get first => ((__x13) => DEVC$RT.cast(__x13, dynamic, V, "CastGeneral", """line 122, column 18 of dart:collection/maps.dart: """, __x13 is V, false))(_map[_map.keys.first]);
- V get single => ((__x14) => DEVC$RT.cast(__x14, dynamic, V, "CastGeneral", """line 123, column 19 of dart:collection/maps.dart: """, __x14 is V, false))(_map[_map.keys.single]);
- V get last => ((__x15) => DEVC$RT.cast(__x15, dynamic, V, "CastGeneral", """line 124, column 17 of dart:collection/maps.dart: """, __x15 is V, false))(_map[_map.keys.last]);
+ V get first => ((__x13) => DEVC$RT.cast(__x13, dynamic, V, "CompositeCast", """line 122, column 18 of dart:collection/maps.dart: """, __x13 is V, false))(_map[_map.keys.first]);
+ V get single => ((__x14) => DEVC$RT.cast(__x14, dynamic, V, "CompositeCast", """line 123, column 19 of dart:collection/maps.dart: """, __x14 is V, false))(_map[_map.keys.single]);
+ V get last => ((__x15) => DEVC$RT.cast(__x15, dynamic, V, "CompositeCast", """line 124, column 17 of dart:collection/maps.dart: """, __x15 is V, false))(_map[_map.keys.last]);
  Iterator<V> get iterator => new _MapBaseValueIterator<V>(_map);
 }
  class _MapBaseValueIterator<V> implements Iterator<V> {final Iterator _keys;
  final Map _map;
  V _current = null;
  _MapBaseValueIterator(Map map) : _map = map, _keys = map.keys.iterator;
  bool moveNext() {
 if (_keys.moveNext()) {
-_current = ((__x16) => DEVC$RT.cast(__x16, dynamic, V, "CastGeneral", """line 144, column 18 of dart:collection/maps.dart: """, __x16 is V, false))(_map[_keys.current]);
+_current = ((__x16) => DEVC$RT.cast(__x16, dynamic, V, "CompositeCast", """line 144, column 18 of dart:collection/maps.dart: """, __x16 is V, false))(_map[_keys.current]);
  return true;
 }
  _current = null;
  return false;
 }
  V get current => _current;
 }
  abstract class _UnmodifiableMapMixin<K, V> implements Map<K, V> {void operator []=(K key, V value) {
 throw new UnsupportedError("Cannot modify unmodifiable map");
 }
@@ skipping 119 matching lines @@
  while (hasNextKey && hasNextValue) {
 map[keyIterator.current] = valueIterator.current;
  hasNextKey = keyIterator.moveNext();
  hasNextValue = valueIterator.moveNext();
 }
  if (hasNextKey || hasNextValue) {
 throw new ArgumentError("Iterables do not have same length.");
 }
 }
 }