Add support for "??" to summaries.

pkg/analyzer/lib/src/summary/link.dart

Index: pkg/analyzer/lib/src/summary/link.dart
diff --git a/pkg/analyzer/lib/src/summary/link.dart b/pkg/analyzer/lib/src/summary/link.dart
index 6b2e1d711bb79d8355471647a36aa08ece60df86..f4486bb0e969784fc79068f47f3075497563bd13 100644
--- a/pkg/analyzer/lib/src/summary/link.dart
+++ b/pkg/analyzer/lib/src/summary/link.dart
@@ -2265,6 +2265,9 @@ class ExprTypeComputer {
         case UnlinkedExprOperation.pushParameter:
           stack.add(_findParameterType(_getNextString()));
           break;
+        case UnlinkedExprOperation.ifNull:
+          _doIfNull();
+          break;
         default:
           // TODO(paulberry): implement.
           throw new UnimplementedError('$operation');
@@ -2404,6 +2407,14 @@ class ExprTypeComputer {
     }());
   }
 
+  void _doIfNull() {
+    DartType secondType = stack.removeLast();
+    DartType firstType = stack.removeLast();
+    DartType type = _leastUpperBound(firstType, secondType);
+    type = _dynamicIfNull(type);
+    stack.add(type);
+  }
+
   void _doInvokeConstructor() {
     int numNamed = _getNextInt();
     int numPositional = _getNextInt();