Add optional message argument to assert statements in the VM.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index 4ebafa283ba85f6787b024ad754c81440bd2552f..16d8ec20652c339b00f12a130a63fbb42c47817b 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -70,6 +70,7 @@ DEFINE_FLAG(bool,
             assert_initializer,
             false,
             "Allow asserts in initializer lists.");
+DEFINE_FLAG(bool, assert_message, false, "Allow message in assert statements");
 
 DECLARE_FLAG(bool, profile_vm);
 DECLARE_FLAG(bool, trace_service);
@@ -9164,32 +9165,91 @@ AstNode* Parser::ParseAssertStatement(bool is_const) {
   const TokenPosition condition_pos = TokenPos();
   if (!I->asserts()) {
     SkipExpr();
+    if (FLAG_assert_message && (CurrentToken() == Token::kCOMMA)) {
+      ConsumeToken();
+      SkipExpr();
+    }
     ExpectToken(Token::kRPAREN);
     return NULL;
   }
-  AstNode* condition = ParseAwaitableExpr(kAllowConst, kConsumeCascades, NULL);
+
+  BoolScope saved_seen_await(&parsed_function()->have_seen_await_expr_, false);
+  AstNode* condition = ParseExpr(kAllowConst, kConsumeCascades);
   if (is_const && !condition->IsPotentiallyConst()) {
     ReportError(condition_pos,
                 "initializer assert expression must be compile time constant.");
   }
   const TokenPosition condition_end = TokenPos();
+  AstNode* message = NULL;
+  TokenPosition message_pos = TokenPosition::kNoSource;
+  if (FLAG_assert_message && CurrentToken() == Token::kCOMMA) {
+    ConsumeToken();
+    message_pos = TokenPos();
+    message = ParseExpr(kAllowConst, kConsumeCascades);
+    if (is_const && !message->IsPotentiallyConst()) {
+      ReportError(
+          message_pos,
+          "initializer assert expression must be compile time constant.");
+    }
+  }
   ExpectToken(Token::kRPAREN);
 
+  if (!is_const) {
+    // Check for assertion condition being a function if not const.
+    ArgumentListNode* arguments = new (Z) ArgumentListNode(condition_pos);
+    arguments->Add(condition);
+    condition = MakeStaticCall(
+        Symbols::AssertionError(),
+        Library::PrivateCoreLibName(Symbols::EvaluateAssertion()), arguments);
+  }
+  AstNode* not_condition =
+      new (Z) UnaryOpNode(condition_pos, Token::kNOT, condition);
+
+  // Build call to _AsertionError._throwNew(start, end, message)
   ArgumentListNode* arguments = new (Z) ArgumentListNode(condition_pos);
-  arguments->Add(condition);
   arguments->Add(new (Z) LiteralNode(
       condition_pos,
-      Integer::ZoneHandle(Z, Integer::New(condition_pos.value(), Heap::kOld))));
+      Integer::ZoneHandle(Z, Integer::New(condition_pos.Pos()))));
   arguments->Add(new (Z) LiteralNode(
       condition_end,
-      Integer::ZoneHandle(Z, Integer::New(condition_end.value(), Heap::kOld))));
+      Integer::ZoneHandle(Z, Integer::New(condition_end.Pos()))));
+  if (message == NULL) {
+    message = new (Z) LiteralNode(condition_end, Instance::ZoneHandle(Z));
+  }
+  arguments->Add(message);
   AstNode* assert_throw = MakeStaticCall(
       Symbols::AssertionError(),
-      Library::PrivateCoreLibName(is_const ? Symbols::CheckConstAssertion()
-                                           : Symbols::CheckAssertion()),
-      arguments);
+      Library::PrivateCoreLibName(Symbols::ThrowNew()), arguments);
 
-  return assert_throw;
+  AstNode* assertion_check = NULL;
+  if (parsed_function()->have_seen_await()) {
+    // The await transformation must be done manually because assertions
+    // are parsed as statements, not expressions. Thus, we need to check
+    // explicitely whether the arguments contain await operators. (Note that
+    // we must not parse the arguments with ParseAwaitableExpr(). In the
+    // corner case of assert(await a, await b), this would create two
+    // sibling scopes containing the temporary values for a and b. Both
+    // values would be allocated in the same internal context variable.)
+    //
+    // Build !condition ? _AsertionError._throwNew(...) : null;
+    // We need to use a conditional expression because the await transformer
+    // cannot transform if statements.
+    assertion_check = new (Z) ConditionalExprNode(
+        condition_pos, not_condition, assert_throw,
+        new (Z) LiteralNode(condition_pos, Object::null_instance()));
+    OpenBlock();
+    AwaitTransformer at(current_block_->statements, async_temp_scope_);
+    AstNode* transformed_assertion = at.Transform(assertion_check);
+    SequenceNode* preamble = CloseBlock();
+    preamble->Add(transformed_assertion);
+    assertion_check = preamble;
+  } else {
+    // Build if (!condition) _AsertionError._throwNew(...)
+    assertion_check = new (Z)
+        IfNode(condition_pos, not_condition,
+               NodeAsSequenceNode(condition_pos, assert_throw, NULL), NULL);
+  }
+  return assertion_check;
 }