Remove number from operator mismatch message.

pkg/front_end/messages.yaml

 # Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # Each entry in this map corresponds to a diagnostic message. Ideally, each
 # entry contains three parts:
 #
 # 1. A message template (template).
 #
 # 2. A suggestion for how to correct the problem (tip).
@@ skipping 28 matching lines @@
 #
 # `#character` a Unicode character.
 #
 # `#unicode` a Unicode short identifier (U+xxxx). We use this to represent code
 #  units or code points.
 #
 # `#name` a name (a string).
 #
 # `#name2` another name (a string).
 #
-# `#number` a number (an integer).
-#
 # `#lexeme` a token. The token's `lexeme` property is used.
 #
 # `#string` a string.
 #
 # `#string2` another string.
 #
 # `#uri` a Uri.
 
 AsciiControlCharacter:
   template: "The control character #unicode can only be used in strings and comments."
@@ skipping 706 matching lines @@
 
 DuplicatedParameterName:
   template: "Duplicated parameter name '#name'."
 
 DuplicatedParameterNameCause:
   template: "Other parameter named '#name'."
 
 EnumConstantSameNameAsEnclosing:
   template: "Name of enum constant '#name' can't be the same as the enum's own name."
 
-OperatorParameterMismatch:
-  template: "Operator '#name' must have exactly #number parameters."
+OperatorParameterMismatch0:
+  template: "Operator '#name' shouldn't have any parameters."
+
+OperatorParameterMismatch1:
+  template: "Operator '#name' should have exactly one parameter."
+
+OperatorParameterMismatch2:
+  template: "Operator '#name' should have exactly two parameters."
+
+OperatorMinusParameterMismatch:
+  template: "Operator '#name' should have zero or one parameter."
+  tip: >-
+    With zero parameters, it has the syntactic form '-a', formally known as 'unary-'.
+    With one parameter, it has the syntactic form 'a - b', formally known as '-'.
 
 SupertypeIsIllegal:
   template: "The type '#name' can't be used as supertype."
 
 SupertypeIsTypeVariable:
   template: "The type variable '#name' can't be used as supertype."
 
 PartOfLibraryNameMismatch:
   template: "Using '#uri' as part of '#name' but its 'part of' declaration says '#name2'."
 
@@ skipping 83 matching lines @@
 
 CantInferPackagesFromManyInputs:
   template: "Cannot infer a .packages file when compiling multiple inputs."
   tip: "Try specifying the file explicitly with the --packages option."
 
 PackageNotFound:
   template: "Could not resolve the package '#name' in '#uri'."
 
 InvalidPackageUri:
   template: "Invalid package Uri '#uri':\n  #string."