Finish strong mode cleaning of dart2js.

pkg/compiler/lib/src/types/dictionary_type_mask.dart

 // 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.
 
 part of masks;
 
 /**
  * A [DictionaryTypeMask] is a [TypeMask] for a specific allocation
  * site of a map (currently only internal Map class) that is used as
  * a dictionary, i.e. a mapping from a set of statically known strings
@@ skipping 35 matching lines @@
         other.typeMap.keys.every(
             (k) => typeMap.containsKey(k) && typeMap[k] == other.typeMap[k]);
   }
 
   TypeMask intersection(TypeMask other, ClosedWorld closedWorld) {
     TypeMask forwardIntersection = forwardTo.intersection(other, closedWorld);
     if (forwardIntersection.isEmptyOrNull) return forwardIntersection;
     return forwardIntersection.isNullable ? nullable() : nonNullable();
   }
 
-  TypeMask union(other, ClosedWorld closedWorld) {
+  TypeMask union(dynamic other, ClosedWorld closedWorld) {
     if (this == other) {
       return this;
     } else if (equalsDisregardNull(other)) {
       return other.isNullable ? other : this;
     } else if (other.isEmptyOrNull) {
       return other.isNullable ? this.nullable() : this;
     } else if (other.isDictionary) {
       TypeMask newForwardTo = forwardTo.union(other.forwardTo, closedWorld);
       TypeMask newKeyType = keyType.union(other.keyType, closedWorld);
       TypeMask newValueType = valueType.union(other.valueType, closedWorld);
@@ skipping 28 matching lines @@
   bool operator ==(other) => super == other;
 
   int get hashCode {
     return computeHashCode(allocationNode, isNullable, typeMap, forwardTo);
   }
 
   String toString() {
     return 'Dictionary mask: [$keyType/$valueType with $typeMap] type: $forwardTo';
   }
 }