VM: Fix bug with ??= expressions using await.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index eb3bd3d1a1a859ad017b5207326ff41c71220a5e..d481a8f9dcd89971af5b26cf466bd895e9ebba18 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -10566,6 +10566,25 @@ AstNode* Parser::OptimizeBinaryOpNode(intptr_t op_pos,
       }
     }
   }
+  if (binary_op == Token::kIFNULL) {
+    // Handle a ?? b.
+    const intptr_t no_pos = Scanner::kNoSourcePos;
+    LetNode* result = new(Z) LetNode(no_pos);
+    LocalVariable* left_temp = result->AddInitializer(lhs);
+    LiteralNode* null_operand =
+        new LiteralNode(no_pos, Instance::ZoneHandle(Z));
+    LoadLocalNode* load_left_temp = new(Z) LoadLocalNode(no_pos, left_temp);
+    ComparisonNode* null_compare =
+        new ComparisonNode(no_pos,
+                           Token::kNE_STRICT,
+                           load_left_temp,
+                           null_operand);
+    result->AddNode(new(Z) ConditionalExprNode(op_pos,
+                                               null_compare,
+                                               load_left_temp,
+                                               rhs));
+    return result;
+  }
   return new(Z) BinaryOpNode(op_pos, binary_op, lhs, rhs);
 }
 
@@ -10737,10 +10756,22 @@ AstNode* Parser::CreateAssignmentNode(AstNode* original,
                              ifnull->right(),
                              left_ident,
                              left_pos);
-    result = new(Z) BinaryOpNode(rhs->token_pos(),
-                                 Token::kIFNULL,
-                                 ifnull->left(),
-                                 modified_assign);
+    const intptr_t no_pos = Scanner::kNoSourcePos;
+    LetNode* let_node = new(Z) LetNode(no_pos);

[hausner, 2015/11/03 19:03:08]

Should be possible to call OptimizeBinaryOpNode() here and re-use that code.

[Florian Schneider, 2015/11/03 19:10:59]

Yes, good point.

+    LocalVariable* left_temp = let_node->AddInitializer(ifnull->left());
+    LiteralNode* null_operand =
+        new LiteralNode(no_pos, Instance::ZoneHandle(Z));
+    LoadLocalNode* load_left_temp = new(Z) LoadLocalNode(no_pos, left_temp);
+    ComparisonNode* null_compare =
+        new ComparisonNode(no_pos,
+                           Token::kNE_STRICT,
+                           load_left_temp,
+                           null_operand);
+    let_node->AddNode(new(Z) ConditionalExprNode(rhs->token_pos(),
+                                                 null_compare,
+                                                 load_left_temp,
+                                                 modified_assign));
+    result = let_node;
   }
   return result;
 }