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

pkg/compiler/lib/src/diagnostics/messages.dart

Index: pkg/compiler/lib/src/diagnostics/messages.dart
diff --git a/pkg/compiler/lib/src/diagnostics/messages.dart b/pkg/compiler/lib/src/diagnostics/messages.dart
index e17157442c72e8251650e482d8a175d88d0797aa..63fa370d6cd2913ce1df8ae2836876abbb91202b 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -326,7 +326,6 @@ enum MessageKind {
   MULTI_INHERITANCE,
   NAMED_ARGUMENT_NOT_FOUND,
   NAMED_FUNCTION_EXPRESSION,
-  NAMED_PARAMETER_WITH_EQUALS,
   NATIVE_NOT_SUPPORTED,
   NO_BREAK_TARGET,
   NO_CATCH_NOR_FINALLY,
@@ -1372,19 +1371,6 @@ main() {
 }"""
           ]),
 
-      MessageKind.NAMED_PARAMETER_WITH_EQUALS: const MessageTemplate(
-          MessageKind.NAMED_PARAMETER_WITH_EQUALS,
-          "Named optional parameters can't use '=' to specify a default "
-          "value.",
-          howToFix: "Try replacing '=' with ':'.",
-          examples: const [
-            """
-main() {
-  foo({a = 1}) => print(a);
-  foo(a: 2);
-}"""
-          ]),
-
       MessageKind.POSITIONAL_PARAMETER_WITH_EQUALS: const MessageTemplate(
           MessageKind.POSITIONAL_PARAMETER_WITH_EQUALS,
           "Positional optional parameters can't use ':' to specify a "