Avoid Object.hashCode on a few objects

pkg/compiler/lib/src/dart_types.dart

Index: pkg/compiler/lib/src/dart_types.dart
diff --git a/pkg/compiler/lib/src/dart_types.dart b/pkg/compiler/lib/src/dart_types.dart
index 8df9c18c1b107b09c162878e219188e5b3720bd7..52621387adf97e00dfe993db4103cdecc56fe05d 100644
--- a/pkg/compiler/lib/src/dart_types.dart
+++ b/pkg/compiler/lib/src/dart_types.dart
@@ -445,6 +445,8 @@ abstract class GenericType extends DartType {
 }
 
 class InterfaceType extends GenericType {
+  int _hashCode;
+
   InterfaceType(ClassElement element,
                 [List<DartType> typeArguments = const <DartType>[]])
       : super(element, typeArguments) {
@@ -503,7 +505,7 @@ class InterfaceType extends GenericType {
     return member;
   }
 
-  int get hashCode => super.hashCode;
+  int get hashCode => _hashCode ??= super.hashCode;
 
   InterfaceType asRaw() => super.asRaw();
 
@@ -825,6 +827,8 @@ class DynamicType extends DartType {
     return visitor.visitDynamicType(this, argument);
   }
 
+  int get hashCode => 91;
+
   String toString() => name;
 }