Add `=` as default-value separator for named parameters.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index cb9c620101ec318ff2baffb48bf4f5a698cea49e..78fe0942664f108d8272055a7e463501973d2fca 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -2184,7 +2184,7 @@ void Parser::ParseFormalParameter(bool allow_explicit_default_value,
     if (params->has_optional_positional_parameters) {
       ExpectToken(Token::kASSIGN);
     } else {
-      ExpectToken(Token::kCOLON);
+      ConsumeToken();

[eernst, 2016/10/11 20:59:50]

It was not obvious to me why this would work, but here's the reason I found: (1)
ConsumeToken() consumes the same token which is tested in line 2178, and (2)
ReportError cannot return. Then we know that the consumed token is '=' or ':',
and that `params->has_optional_named_parameters` is true.

[Lasse Reichstein Nielsen, 2016/10/12 06:10:58]

Correct.
ConsumeToken is used to drop the current token and advance to the next, no
matter what it is. It's only used when we have already tested the current token
somewhere else and know what it is.
The ReportError does unwind the stack to report the error. You could think that
it throws (I actually think it longjumps, but potato-potatoh).

     }
     params->num_optional_parameters++;
     params->has_explicit_default_values = true;  // Also if explicitly NULL.