Implement hashCode on objects stored in a set or used as map keys.

dart/sdk/lib/_internal/compiler/implementation/types/container_type_mask.dart

Index: dart/sdk/lib/_internal/compiler/implementation/types/container_type_mask.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/types/container_type_mask.dart b/dart/sdk/lib/_internal/compiler/implementation/types/container_type_mask.dart
index f74db6e01fb66911c5769bab55b85a189dd4f366..c8c06f5d5938f2694497dd3a4ef50de11ae89a73 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/types/container_type_mask.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/types/container_type_mask.dart
@@ -10,6 +10,8 @@ part of types;
 class ElementTypeHolder {
   // This field will be set after global analysis.
   TypeMask elementType;
+
+  int get hashCode => elementType.hashCode;
 }
 
 /// A [ContainerTypeMask] is a [TypeMask] for a specific allocation
@@ -84,4 +86,8 @@ class ContainerTypeMask extends ForwardingTypeMask {
   String toString() {
     return 'Container mask: $elementType';
   }
+
+  int get hashCode {
+    return computeHashCode(asFlat, allocationNode, allocationElement, holder);
+  }
 }