Update parser to not check that all final fields are initialized for external constructors.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index 1e92e4c0d4553a731497ff796724b1ebdf350dee..539acedc006c566006958c1d9197f82b1519facd 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -1679,6 +1679,13 @@ void Parser::CheckDuplicateFieldInit(intptr_t init_pos,
 }
 
 
+void Parser::CheckNoInitializersInExternal() {
+  if (CurrentToken() == Token::kCOLON) {
+    ErrorMsg("initializer lists invalid for 'external' constructors");
+  }
+}
+
+
 void Parser::ParseInitializers(const Class& cls,
                                LocalVariable* receiver,
                                GrowableArray<Field*>* initialized_fields) {
@@ -1914,7 +1921,12 @@ SequenceNode* Parser::ParseConstructor(const Function& func,
   }
 
   // Now parse the explicit initializer list or constructor redirection.
-  ParseInitializers(cls, receiver, &initialized_fields);
+  if (!func.is_external()) {
+    ParseInitializers(cls, receiver, &initialized_fields);
+  } else {
+    // 'external' constructors should have no initializers.
+    CheckNoInitializersInExternal();
+  }
 
   SequenceNode* init_statements = CloseBlock();
   if (init_statements->length() > 0) {