Add support for "??" to summaries.

R=scheglov@google.com

Review URL: https://codereview.chromium.org/2514353005 .

pkg/analyzer/lib/src/summary/format.fbs

@@ -758,7 +758,14 @@ enum UnlinkedExprOperation : byte {
    * the nth enclosing function element.  Then, push the mth local function of
    * that element onto the stack.
    */
-  pushLocalFunctionReference
+  pushLocalFunctionReference,
+
+  /**
+   * Pop the top two values from the stack.  If the first value is non-null,
+   * keep it and discard the second.  Otherwise, keep the second and discard the
+   * first.
+   */
+  ifNull
 }
 
 /**

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

@@ -2333,6 +2333,13 @@ enum UnlinkedExprOperation {
    * that element onto the stack.
    */
   pushLocalFunctionReference,
+
+  /**
+   * Pop the top two values from the stack.  If the first value is non-null,
+   * keep it and discard the second.  Otherwise, keep the second and discard the
+   * first.
+   */
+  ifNull,
 }
 
 /**

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();

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

@@ -496,6 +496,9 @@ class _ConstExprBuilder {
                       'Unable to resolve constructor parameter: $name'));
           _push(identifier);
           break;
+        case UnlinkedExprOperation.ifNull:
+          _pushBinary(TokenType.QUESTION_QUESTION);
+          break;
         case UnlinkedExprOperation.assignToRef:
         case UnlinkedExprOperation.assignToProperty:
         case UnlinkedExprOperation.assignToIndex:

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

@@ -486,6 +486,8 @@ abstract class AbstractConstExprSerializer {
       operations.add(UnlinkedExprOperation.lessEqual);
     } else if (operator == TokenType.PERCENT) {
       operations.add(UnlinkedExprOperation.modulo);
+    } else if (operator == TokenType.QUESTION_QUESTION) {
+      operations.add(UnlinkedExprOperation.ifNull);
     } else {
       throw new StateError('Unknown operator: $operator');
     }

pkg/analyzer/test/src/summary/resynthesize_common.dart

@@ -2302,6 +2302,12 @@ const vIdentical = (1 == 2) ? 11 : 22;
 ''');
   }
 
+  test_const_topLevel_ifNull() {
+    checkLibrary(r'''
+const vIfNull = 1 ?? 2.0;
+''');
+  }
+
   test_const_topLevel_literal() {
     checkLibrary(r'''
 const vNull = null;

pkg/analyzer/test/src/summary/summary_common.dart

@@ -1755,6 +1755,18 @@ var v = (() {
     ]);
   }
 
+  test_constExpr_binary_qq() {
+    UnlinkedVariable variable = serializeVariableText('const v = 1 ?? 2;');
+    _assertUnlinkedConst(variable.initializer.bodyExpr, operators: [
+      UnlinkedExprOperation.pushInt,
+      UnlinkedExprOperation.pushInt,
+      UnlinkedExprOperation.ifNull
+    ], ints: [
+      1,
+      2
+    ]);
+  }
+
   test_constExpr_binary_subtract() {
     UnlinkedVariable variable = serializeVariableText('const v = 1 - 2;');
     _assertUnlinkedConst(variable.initializer.bodyExpr, operators: [