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 7829fd6649a6beabf94128613d149f589b16b56b..778a338d6c6714bc5b28ccd343b358d368ac2a85 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -71,6 +71,7 @@ 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);
@@ -9189,6 +9190,24 @@ AstNode* Parser::MakeStaticCall(const String& cls_name,
 }
 
 
+AstNode* Parser::MakeAssertCall(TokenPosition begin,
+                                TokenPosition end,
+                                AstNode* message) {
+  ArgumentListNode* arguments = new (Z) ArgumentListNode(begin);
+  arguments->Add(new (Z) LiteralNode(
+      begin, Integer::ZoneHandle(Z, Integer::New(begin.Pos()))));
+  arguments->Add(new (Z) LiteralNode(
+      end, Integer::ZoneHandle(Z, Integer::New(end.Pos()))));
+  if (message == NULL) {
+    message = new (Z) LiteralNode(end, Instance::ZoneHandle(Z));
+  }
+  arguments->Add(message);
+  return MakeStaticCall(Symbols::AssertionError(),
+                        Library::PrivateCoreLibName(Symbols::ThrowNew()),
+                        arguments);
+}
+
+
 AstNode* Parser::ParseAssertStatement(bool is_const) {
   TRACE_PARSER("ParseAssertStatement");
   ConsumeToken();  // Consume assert keyword.
@@ -9196,6 +9215,10 @@ 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;
   }
@@ -9205,23 +9228,36 @@ AstNode* Parser::ParseAssertStatement(bool is_const) {
                 "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 = ParseAwaitableExpr(kAllowConst, kConsumeCascades, NULL);
+    if (is_const && !message->IsPotentiallyConst()) {
+      ReportError(
+          message_pos,
+          "initializer assert expression must be compile time constant.");
+    }
+  }
   ExpectToken(Token::kRPAREN);
 
-  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))));
-  arguments->Add(new (Z) LiteralNode(
-      condition_end,
-      Integer::ZoneHandle(Z, Integer::New(condition_end.value(), Heap::kOld))));
-  AstNode* assert_throw = MakeStaticCall(
-      Symbols::AssertionError(),
-      Library::PrivateCoreLibName(is_const ? Symbols::CheckConstAssertion()
-                                           : Symbols::CheckAssertion()),
-      arguments);
-
-  return assert_throw;
+  if (!is_const) {
+    // Check for being a function if not const.
+    ArgumentListNode* arguments = new (Z) ArgumentListNode(condition_pos);
+    arguments->Add(condition);
+    condition = MakeStaticCall(
+        Symbols::AssertionError(),
+        Library::PrivateCoreLibName(Symbols::CheckAssertion()), arguments);
+  }
+  AstNode* not_condition =
+      new (Z) UnaryOpNode(condition_pos, Token::kNOT, condition);
+  AstNode* assert_throw = MakeAssertCall(condition_pos, condition_end, message);
+  return new (Z)
+      IfNode(condition_pos, not_condition,
+             NodeAsSequenceNode((message == NULL) ? condition_pos : message_pos,
+                                assert_throw, NULL),
+             NULL);
 }