[front_end] Allow "parameters" entries in messages.yaml.

A `parameters` entry in `pkg/front_end/messages.yaml` has the same
meaning as a `parameters` entry in `pkg/analyzer/messages.yaml`.

For now there are no `parameters` entries present. In follow-up CLs, I
will add a script that adds `parameters` entries to
`pkg/front_end/messages.yaml`, and then I will make the `parameters`
entries required. After that, I will make it possible to choose
arbitrary names for parameters; this will bring the front end's
parameter functionality in line with the analyzer's.

Change-Id: I6a6a6964c293f65c67a12739cab05c02ba3bb8e2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/448234
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/front_end/messages.yaml

@@ -133,6 +133,15 @@
 #
 # `#num1%N.M`, `#num2%N.M`, `#num3%N.M`: numbers (doubles) formatted to minimum
 # width N and with M fraction digits.
+#
+# A message declaration may optionally include a map called
+# `parameters`; each map key is of the form `TYPE NAME`, and each map
+# value is a comment describing the parameter.
+#
+# If a diagnostic takes no parameters, it should have an entry of the
+# form `parameters: none`.
+#
+# In the future, the `parameters` entry will be required.
 
 AsciiControlCharacter:
   problemMessage: "The control character #unicode can only be used in strings and comments."

pkg/front_end/test/messages_suite.dart

@@ -446,6 +446,25 @@ class MessageTestSuite extends ChainContext {
             }
             break;
 
+          case 'parameters':
+            switch (value) {
+              case 'none':
+                break;
+              case YamlMap():
+                for (var parameterDoc in value.values) {
+                  if (parameterDoc is! String) {
+                    throw new ArgumentError(
+                      'parameter documentation should be a string: '
+                      '$parameterDoc',
+                    );
+                  }
+                }
+              default:
+                throw new ArgumentError(
+                  'parameters should be a map or `none`: $value.',
+                );
+            }
+
           default:
             unknownKeys.add(key);
         }