Add support for "??" to summaries.

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

 // Copyright (c) 2016, 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.
 
 /**
  * This library is capable of producing linked summaries from unlinked
  * ones (or prelinked ones).  It functions by building a miniature
  * element model to represent the contents of the summaries, and then
  * scanning the element model to gather linked information and adding
  * it to the summary data structures.
@@ skipping 2247 matching lines @@
           stack.add(BottomTypeImpl.instance);
           break;
         case UnlinkedExprOperation.pushLocalFunctionReference:
           int popCount = _getNextInt();
           assert(popCount == 0); // TODO(paulberry): handle the nonzero case.
           stack.add(function.functions[_getNextInt()].type);
           break;
         case UnlinkedExprOperation.pushParameter:
           stack.add(_findParameterType(_getNextString()));
           break;
+        case UnlinkedExprOperation.ifNull:
+          _doIfNull();
+          break;
         default:
           // TODO(paulberry): implement.
           throw new UnimplementedError('$operation');
       }
     }
     assert(intPtr == unlinkedConst.ints.length);
     assert(refPtr == unlinkedConst.references.length);
     assert(strPtr == unlinkedConst.strings.length);
     assert(assignmentOperatorPtr == unlinkedConst.assignmentOperators.length);
     assert(stack.length == 1);
@@ skipping 119 matching lines @@
           } else {
             // Method tear-off
             return element.type;
           }
         }
       }
       return DynamicTypeImpl.instance;
     }());
   }
 
+  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();
     // TODO(paulberry): don't just pop the args; use their types
     // to infer the type of type arguments.
     stack.length -= numNamed + numPositional;
     strPtr += numNamed;
     EntityRef ref = _getNextRef();
     ClassElementForLink_Class element =
         unit.resolveConstructorClassRef(ref.reference).asClass;
@@ skipping 2662 matching lines @@
    * there are no type parameters in scope.
    */
   TypeParameterizedElementMixin get _typeParameterContext;
 
   @override
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 
   @override
   String toString() => '$enclosingElement.$name';
 }