[ast] Mark temporaries as maybe-assigned by default.

src/parsing/parser.cc

Index: src/parsing/parser.cc
diff --git a/src/parsing/parser.cc b/src/parsing/parser.cc
index b9d26995f1b1e19078b93de222c03bd16f08de64..d0e0e999213b83c013713dce70727e3a84d175d2 100644
--- a/src/parsing/parser.cc
+++ b/src/parsing/parser.cc
@@ -2991,7 +2991,7 @@ Block* Parser::BuildRejectPromiseOnException(Block* inner_block) {
         Context::ASYNC_FUNCTION_PROMISE_CREATE_INDEX,
         new (zone()) ZoneList<Expression*>(0, zone()), kNoSourcePosition);
     Assignment* assign_promise = factory()->NewAssignment(
-        Token::INIT, factory()->NewVariableProxy(PromiseVariable()),
+        Token::ASSIGN, factory()->NewVariableProxy(PromiseVariable()),

[neis, 2017/02/15 13:44:33]

This was a bug because we potentially assign twice to the promise variable. 
With this CL, the change from INIT to ASSIGN is purely cosmetic since the
temporary is now already marked as assigned.

         create_promise, kNoSourcePosition);
     set_promise =
         factory()->NewExpressionStatement(assign_promise, kNoSourcePosition);